Recommended modifications/maintenance for 94-95 GM 6.5L electronic
fuel injection (EFI) diesels, in relative order, by buddy, version 2.71
Much of this information is applicable to older and newer 6.5 diesels. Older mechanical injection trucks can
benefit from all but the EFI and PCM info, and newer OBD2 vehicles may ignore the OPS info and PCM
upgrade method, and already have pieces of the cooling and air cleaner improvements. This is not a full
diagnostics reference; check out helpful online forums. I recommend the real GM Service Manual and Diesel
Supplement, GM’s “6.5L Diesel Electronic Fuel Injection” training book, or ALLDATAdiy subscription. I
have no affiliation with vendors mentioned, other than being a repeat customer to most.
(1) First and foremost, the vehicle needs to be in good working order, set up properly. In order to ensure that it
is, a scanner is extremely useful. Engh Motors sells scanner software called GMTDScan Tech that works for
1994 and 1995 OBD1 6.5L diesel engines. Make sure your Injection Pump (IP) is properly timed, calibrated
and well lubricated. With the engine at warm idle, make sure the fuel rate is about 6-10mm3 and measured
timing is very close to desired timing. If the fuel rate is 0-4mm3, you either performed an optic bump or this IP
was not calibrated by the rebuilder. Having a low idle fuel rate doesn’t actually mean you are using less fuel,
just that the Powertrain Control Module (PCM) is confused as to what 1mm3 of fuel actually means in terms of
the pulse width it commands. This can cause poor idle and shifting quality, with loping, fishbites, stalling, limp
mode and an overly sensitive fuel pedal. This can be corrected by repositioning the Optical Sensor (OS) within
the IP. Remove a snap ring around the OS connector, remove the IP’s top lid (which you might have to do first
to get the snap ring), loosen the T-40 screw holding the OS in place, and move the OS (lower rectanglular
piece) towards driver side (US) while keeping the screw centered, to increase the commanded fuel rate at idle.
An idle fuel rate too high is not that bad, except you might not be able to belch as much black smoke and
slightly less top end power. Either condition can also be offset with programming, what I refer to as Optic
Bump normalization. On the OS topic, these years had a “black box” electro-magnetic/radio frequency
interference (EMI/RFI) filter between the OS and engine harness. Filter degradation often leads to fishbiting or
unexplained codes. It is commonly removed with no ill effects, by connecting the engine harness directly to the
OS; otherwise there are replacement filters.
Another adjustment that can be made on the IP is an allen screw valve in the lower IP inlet fitting (which is
pictured on page 10 with the fuel filter manager). This transfer pump pressure relief valve screw changes at
what pressure the IP’s fill/spill chamber fuel pressure bleeds to the transfer pump input. It allows the IP to
maintain input pressure even when the lift pump is not providing pressure from the fuel tank. Tightening this
screw for more input pressure can help maintain higher fuel rates and proper injection timing, but could prevent
the IP from being able to run when the lift pump fails.
Left: Two parts of OS exposed after removing upper intake and
top of IP. The lower OS piece will move side to side after
loosening the T-40 bolt shown.
Below: OS filter above the IP, below upper intake plenum, and
pictured separately removed from the engine bay.
On every fuel fill-up of #2 ULSD add a lubricant, and not just any off the shelf diesel lubricity additive. Many
of the popular additives (i.e. Power Service, Howes) fail to show enough improvement in wear scar testing for
our IPs. Most are only good for #1 LSD fuel or newer systems built for ULSD. 2% biodiesel to B5 fuel is the
best option, or add a gallon of whole biodiesel for 20-40 gallons of ULSD. I do not recommend using higher
than 5% biodiesel concentration, absolute max biodiesel concentration I’d consider is B20. Simple ash-less,
TCW3 2-Cycle oil is an effective additive, recommended max ratio of 200:1, 0.5% concentration, about 2oz for
every 3 gallons. More than an ounce per gallon could make fuel too dark for OS to function, and 400:1 may
work just as well, but 200:1 was tested. Others use Non-Detergent SAE30 motor oil with success, an ounce per
gallon. Used motor oil is a bad idea with the DS4 IP, too dark for the OS, and ineffective for lubricity, with too
much water, acid, and contaminants. On the subject of additives, a cetane improver such as Amsoil Cetane
Boost or Red Line RL-3 is a good idea, because the US has low cetane fuel, other additives are not as effective.
You must also ensure the IP is correctly timed, which is not done by just looking at the scanner data, because
desired and measured timing can be reporting good values even with the IP in the wrong position. The IP
timing procedure results in the PCM storing a Top Dead Center Offset (TDCO) value, which defines how it
correlates OS timing signals to Crankshaft Position Sensor (CPS) signals. You don’t know that your IP is timed
correctly by looking at the reported TDCO value, because it’s a value saved in the PCM until the TDCO Learn
is initiated with a scanner. The IP, OS, CPS, timing chain, PCM, ect... can be changed and it still has the last
value stored in that PCM. This is very common, and to ensure it is timed correctly you must run the TDCO
There can be a significant power difference between a positive and negative TDCO value, even though it may
only be a 1 degree difference in base timing, and the PCM advances the timing to a programmed position
regardless. This is likely because changing IP position may increase IP housing and transfer pressures and ride
the cam ring at a different rise rate when metering fuel, so that the correct volume or more fuel is metered.
Although the scanner displays camshaft timing for actual/measured/desired timing, crankshaft timing is simply
double the reported value, so 1 cam degree change is 2 crank degrees. When changing TDCO from -0.5 to
-1.94 you are changing base timing from 7° to almost 8° crank advance. So 2mm of rotation is about 0.5 cam
degrees, which is 1 crank degree. Some scanners report “actual injection pump timing”, which will be closer to
the crankshaft timing when the engine is operating at advance greater than the physical position of the IP.
The advance can be less than the physical position of the IP, when the stepper motor retards timing from its
nominal position. If you are having an extremely difficult time setting the TDCO or getting strangely low
timing advance at idle then it is possible you have improperly installed the CPS out of rotation. That can
happen if the mounting tab breaks off, or the metal tab can pop off some models and put back on 90°, 180°, or
270° wrong. If you have OS codes (DTC17 or 18), removing the OS and cleaning the eye with acetone and a
soft cloth may resolve them, especially if using bio fuels. Otherwise the OS itself can be replaced and
calibrated as detailed on the first page.
Learn and see if it is within tolerance afterwards. I
like between -0.75 and -1.50. Factory timing placed
the TDCO between -0.25 and -0.75, but the PCM’s
tolerances are from +2.02 to -2.02 before a DTC88 is
reported (unless custom tuned). OBD2 vehicles have
a +/- 2.5 TDCO limit. The TDCO is a crank degree
value from a nominal 25.7°, determined by checking
the base timing value when the stepper motor is
incremented to its most retarded position. The base
timing will be seen when commanding Time Set on
the scanner before commanding the TDCO Learn. To
the right you can see how the IP can be set when
installing it before running the timing procedure. The
IP flange edge next to the top nut should be slightly
towards the driver side of the timing cover wedge.
To time the IP, you will need a few tools and procedures below. This assumes you have the single t-stat housing; with
double housings it takes a bit more work for access, but the procedure is the same. The wrenches required to perform the procedure
on 1996+ thermostat crossovers are significantly different, need to be like the official timing wrenches.
1) The right scanner, GMTDScan Tech, Tech1 or Tech2 (Snap-On scanners display incorrect data)
2) IP nut wrench; A 12-point, 15mm half-moon wrench (~9” long), or an S-bend wrench. There are official
IP nut wrenches (Kent-Moore J-41711 and J-41089, or OTC-6087) that cost $100+.
3) A way to turn and stabilize the IP. Turning the IP can be done with a large oil filter or cannon plug
pliers that fit around the “neck” of the IP, between it’s mounting flange and main body. The official turning
wrench is also $100+ (Kent-Moore J-29872-A or OTC-6083). I have used the oil filter pliers, but also made
a tool for turning the IP. The tool is pictured below, just a 3/4” to 1” wide, 3/16” to 1/4” thick metal bar
with 3/8” threads tapped ~2.2” apart to line up with holes in the IP mounting flange for the official timing
tool; 3/4” long 3/8” bolts are threaded in.
a) Warm up the engine over 170F, and with the scanner verify there are no DTCs. If you have a DTC88 for
timing/TDCO, clear it before running the “TDCO Learn”. If you have other DTCs, such as DTC 17, 18, 19
for the OS or CPS you need to resolve them first.
b) While at normal idle, the lowest idle you have available, nearest to 600rpm, command the “Time Set” to
“On” and observe the actual timing value. The value should be approximately 3.5° degrees, while the
desired timing is at 0°. The value may jump around a lot, but you want an average value of 3.5° to 3.8°.
Command the “Time Set” to “Off” after about 20 seconds. I have found that mine will sometimes go lower
after performing the TDCO Learn, so if it’s a little high continue anyway. The value you see is your “base
timing”, where the IP Stepper Motor has incremented as low as possible.
c) Record the current TDCO value, and then command the “TDCO Learn” to “On” and observe the TDCO
value. This will time out after about 25 seconds, or you can command the “TDCO Learn” to “Off” and it
should store the value on the screen at the time. Occasionally the value may continue to bounce around
after commanding it off, and time out later. You can run the TDCO Learn again if it moved to an undesired
value. Compare to the previous value you recorded to see if your IP was properly timed before. I prefer the
TDCO value to be between -0.75 and -1.5. If you are getting -2.02 or +2.02 that is just the limit the scanner
displays, will set a DTC88, and you could be quite far out of tolerance. The “Time Set” should give you an
idea of how far out you are, if you run it again. A “Time Set” base timing value over 4° will go to -2.02 and
any value under about 2.5° will result in a +2.02. If the base timing value is just over 4°, and you are aiming
for a TDCO value of -1.94 or thereabouts you can run the “TDCO Learn” again and hold the fuel pedal at
1000-2000rpm. This can get you from -2.02, down to -1.85 or -1.94 for you to command the TDCO Learn
“Off”. OBD2 trucks can set the TDCO down to -2.5, likely because those trucks also set high idle when
learning the TDCO and OBD1 trucks do not. Increasing RPM also increases IP transfer pressure which
affects the advance piston, working against the stepper motor, which attempts to retard timing as much as
possible. OBD2 TDCO tables may be offset to compensate for the difference in IP position.
d) Up to now you’ve just verified if your timing was set properly in the past. To modify the TDCO value
you need to physically turn the IP. Always shut the engine off prior to loosening the IP nut(s). To get a less
negative value the IP needs to turn towards the passenger side; more negative turn towards the driver side.
With the engine off, scribe a line on the timing cover where the IP is currently set, then loosen the IP nuts
with the 12-point, 15mm half-moon wrench. The lower nuts are difficult to get to and have limited space to
turn a wrench. It takes some finesse to get the wrench on the nuts, which you’ll have to do 2 or 3 times to
get the nut loose enough. On the passenger side you may need to unclip the turbo oil supply hose from the
coolant crossover. Once you get it figured out and done once it gets much simpler. To turn and hold the IP
you can use the large oil filter pliers, or a tool you fashion. The IP will provide resistance to turning so you
will have to hold it while you tighten the top IP nut. Each millimeter you turn the IP will change the TDCO
value by about 0.7. Leave the bottom two nuts loose while you check the TDCO value by repeating steps
(a) to (c). Repeat step (d) with just the top nut until you achieve the desired TDCO value.
e) Once you achieve the desired TDCO value then shut the engine off and tighten the lower two IP nuts.
Once all IP nuts are tight, I recommend running steps (a) to (c) one more time to verify the IP did not move
while tightening the lower nuts, it happens.
The snapshot below from GMTDScan Tech is a typical example of how the IP could be way too “advanced”
towards the driver side, so much so that the “base timing” seen in Time Set is 9.3°. It is supposed to be
3.5°, but the PCM has an old stored TDCO value of -0.88 that is within tolerance. When I actually run the
TDCO Learn it will set DTC88 and TDCO value will show -2.02. The same concept could happen in a way
too “retarded” manner. This would not set any other DTC and could never be noticed without the Time
Set or TDCO Learn if the desired timing at idle is over 9.3°. The stepper motor DTC34 for timing is not set
until there is more than 5° difference between desired and measured timing. There are some stock
programs with as low as 4.5° timing where this situation could be seen just by looking at the scanner, but
would still not set a code. If you do have a 4.5° idle timing PCM program this error would probably make
the truck much faster off the line, since idle timing between 8° and 11° is a lot more responsive.
However, this is not good for general reliability and efficiency. It could be a free performance upgrade for
some, but idle timing that advanced is not good with a functional EGR valve.
(2) Remote mounted Fuel Solenoid Driver (FSD), a.k.a. Pump Mounted Driver (PMD), on heatsink, using
quality extension cable, silver or copper thermal grease between FSD and heatsink, tighten down, and
weatherize it with a bead of silicone around the outer edge of FSD. Locate the FSD away from engine heat.
FSDs, like any electronics, don’t like to be hot, but they also don’t like to be very cold. Cycling between cold
and hot causes FSD failure, but the solutions center around removing heat. People in very cold climates may
experience higher failure rates in the winter, when keeping them from freezing would help. The idea is to limit
the lower and upper temperature to limit the temperature swings on each warm up and cool down cycle. Leave
the existing ground wire attached to the top of the IP. PMDcable website sells quality 6’ PMD cables.
When mounting the FSD try grounding the heatsink through mounting hardware or by ground strap. Also
ensure the FSD’s integral heatsink has continuity to the external heatsink, so I don’t use thermal pads that
disrupt electrical continuity. I even sanded away the anodized coating of the FSD sink surface that disrupts
continuity. There is thermal grease that claims electrical conductivity that may be best, but I use Arctic Silver
thermal grease. This will ensure the FSD has a short path to ground if it tries to offload excess voltage or EMI.
The PMD was designed with the coating and thermal pad to prevent grounding of its own sink. The base
(power) of the transistors are common to their housings, which means if it touches the PMD chassis/sink and
the sink was grounded then it would let the smoke out (fry). The transistors have isolation mounts, so do not
contact the sink by design, but one in several thousand could fail in time. To maintain multiple layers of
protection, the thermal pad could be used and only scratch off a small channel in the anodized coating; place a
stripped end of a wire between the pad and the FSD’s exposed sink metal. Then run that wire to ground with a
1-amp fuse in line for the 0.01% chance the transistor tears through its isolation pad and shorts to the FSD sink.
I also believe FSDs are sensitive to voltage problems in the vehicle. Jump starter chargers, 50+ amp ones,
should not be used on a 6.5L EFI diesel. These are somewhat unregulated and may spit out 18+ VDC which
can harm electronic devices. The PCM and sensors have power conditioning circuits, but the FSD does not.
The FSD gets straight ignition voltage for primary power. Same goes for the Glow Plug (GP) Controller and
GPs, which can be damaged from over voltage. Shocking or stressing a component often does not lead to
immediate failure; it begins degradation, which worsens with use. This is why you might have FSD failures and
GP system issues months after a stint of alternator or battery problems. If batteries are low, slow charge them if
possible, or if stranded jump the truck with a normally idling vehicle after charging for 15-20 minutes. You can
even leave a slow charger (like 6 amps) on while cranking for a little help if no other vehicle is available. You
might have less starting issues with larger gauge battery/starter cables (available at ptwiringsolutions website),
since it increases cranking RPMs to significantly improve starting. FSD output can be tested using a digital
volt/multi-meter, to diagnose no-start or surging idle condition; some diagnostics are at the end of this list.
(3) Oil Pressure Switch/Sender (OPS) relay modification, to take the load of the Lift Pump (LP) off of the OPS,
because it burns the contacts within the OPS and causes the LP to stop working during engine operation. On
the next page I provide a wiring diagram to add a relay, and also add fuel priming during the GP / Wait to Start
(WTS) time, like OBD2 trucks have for easier starting. Something that improves access to the OPS, and
simplifies wire routing is an OPS extension hose. PMDcable sells nice 6” or longer extensions. The extension
is essentially a 1/4” grease/hydraulic hose with 1/4” male and female pipe thread fittings. Additionally the OPS
requires its threads to be grounded for the oil pressure gauge sender portion to work, which the crimped on
fittings do by intentionally tapping into the hose’s internal steel braiding. When purchasing an OPS, I would
only get the AC Delco part # D1808A.
Keeping the LP power tied to OPS output is to ensure the LP does not continue pumping in case of an accident
when the fuel line at the engine could be severed. The engine dies and the LP would stop pumping because the
OPS would lose oil pressure. If you tie LP power to the ignition, you must consider it will always be running
when the ignition is in the On/Run position. Also, if you want the LP to run with the ignition, you should still
use a relay, so the current for the LP does not pass through the ignition switch. For monitoring voltage to the
LP with the engine running, you can probe the under-dash OBD1 diagnostic connector. In 94 contact “G” has
LP power, contact “F” in 95. Applying voltage to these contacts, which go to the LP relay’s normally closed
contact, will also power the LP.
(4) Change exhaust crossover to mandrel bent pipe and the turbo downpipe to 3" mandrel bent pipe (the
downpipe will usually come with 4" exhaust kits, such as Diamond Eye). The stock crossover pipe is dual
layered and often times the inside layer crushes internally while the outside looks fine. You can identify the
stock crossover by its crinkle bends, which look pleated. The stock downpipe is a discontorted crunched up
mess. The 4” exhaust systems do not typically include the crossover pipe.
(5) Boost gauge (20-30psi range), Exhaust Gas Temperature (EGT) gauge (1400-1600F range) and a
transmission fluid temp gauge (around 260F range). The next most useful gauge may be fuel pressure (15psi
range) at the IP inlet. That is to diagnose low power, fishbiting, stalls, and prevent IP damage when it pulls a
vacuum to suck fuel from the tank. Tranny temp gauge not as important with a manual tranny. The gauge
ranges are not max operating parameters. Transmission oil should be kept below 220F if possible. You should
not go over 1300F EGT pre-turbo, or sustain more than 1200F for very long. We shouldn’t normally push more
than 15psi of boost, for risk of blowing head gaskets or worse. However, with the right turbo, exhaust, head
studs, injectors and PCM tuning, 20-25psi of boost can help make a lot of power.
(6) PCM programming upgrades like economy or super stock, that provide improved efficiency, throttle
response and overall driver satisfaction. This could be because you’re not adding performance modifications, or
the truck is for someone you don’t want hot-rodding, or won’t be towing. In 94 and 95 the brain of the PCM
was located on removable Programmable Read-Only Memory (PROM). So an upgrade is a simple PROM
swap that does not require a timing or anti-theft learn procedure.
(7) Upgrade the exhaust system to 4" from the turbo down pipe back (this is helpful at any earlier point); a
muffler is recommended. For a quiet exhaust I recommend a three chambered muffler, or you could try two
straight through mufflers in series if you have them laying around. There is a difference between a straight 3"
system all the way back and upgrading to 4" from the downpipe back, stock is a mere 2.75". Fluid dynamics
does not follow a weakest link in the chain theory. The longer the smallest diameter pipe is, the more
backpressure it will create. Pipe alone causes backpressure, defined by cross sectional area and length.
Turbochargers work off of pressure differential, so the less backpressure on the exhaust pipe, the less drive
pressure needed to make the same boost. That is more efficiency and power from less parasitic backpressure on
the engine's exhaust stroke. It also means faster spooling and higher peak boost pressures, which is not as
necessary on a GM turbo as larger ones, but also lowers intake air temperature (IAT) and EGT.
(8) Update the air filter box to the K47 cylinder air filter box from 1997+ 6.5 trucks with an AFE MagnumFlow
(9) Make sure your intake is free to flow. Those with EGR setups, but not concerned with inspection
requirements, can change out their lower and upper intake for one with no restrictive EGR plumbing. Also
many of the early year non-EGR “F” engines had the restriction plates in the upper plenum, without the extra
channels inside the lower manifold. The upper plenum you can simply cut out the extra restricting material, and
Pro 5R air filter (10-10005). Use a 3" 90 ̊ PVC street elbow
(male and female end) to connect to the factory rubber boot.
This setup is pictured to the right. This can improve efficiency
and power while lowering IATs and EGTs. The AFE filter is
a 5-layer synthetic dry element that can be cleaned/washed for
many cycles, uses polyurethane ends, flows great and no
oiling. If still there, remove the plastic snorkel that travels
inside the fender from the air box interface to the front of the
fender. No flat panel filter will be adequate. The thick plastic
K47 air cleaner box offers durability, stock appearance, and
insulation from engine bay heat. It can be sourced from
salvage yards, or online at Car-Part.com, make sure to get the
fender interface piece with it, because it differs from the flat
box. You can also get large 4” flange universal Amsoil
NanoFiber cone filters that fit in the K47 or custom enclosure.
if it is an EGR upper plenum make and bolt down a block off plate. If not actually changing out the lower
intake manifold from an EGR setup it would be best to plug the center exhaust gas tower with an appropriate
sized freeze plug a bit larger than the approximately 1.75” (45 mm) diameter hole. The exhaust gases travel
through ports in the heads, a channel inside the lower intake and up this central tower to the upper plenum
where the EGR valve resides on top.
The ports in the heads can be blocked with a non-EGR gasket, otherwise the tower should be plugged so
exhaust cannot leak between the lower and upper intakes. There is and should have been a gasket for the EGR
tower between the upper and lower intakes, but it can leak and better to just plug the tower in the lower intake.
After blocking ports at the heads or plugging the tower the EGR valve could be left in place for a stock look.
Most people request that EGR DTCs be disabled even if the system is left in place.
The pictures below show the upper “S” intake, with the extra restrictive plates outlined to cut. Some non-EGR
“F” intakes also had this restriction in them, but with no holes for the EGR valve. So even if it is an “F” intake
it would be best to check. The lower “S” intake is pictured to the right.
(10) Upgrade the LP to one that can provide 12-14psi fuel pressure at 50gph or better, such as the Delphi
FP953, FASS, or Airdog LPs, each of which requires custom installations easy enough to manage with the right
fittings. PMDcable sells a convenient Walbro FRC10 capable of 16psi at 50gph, includes the OEM electrical
connector, has voltage protection, is flow on fail and safe for all fuel types. This upgrade is of importance when
getting PCM programs with greater fuel output. GM technical data shows the IP should have 6-9psi at its inlet
for proper operation, but stock on many trucks pressure is only 5-7psi at idle, pre-filter from the drain port. The
5 micron filter and restrictive Fuel Filter Manager (FFM) housing between the LP and the IP will drop fuel
pressure ~30%. Under load with a power PCM PROM you can drop a healthy system down to 0-1psi pressure,
even vacuum, and you lose power. For the DS4 IP, it should be set up to have 6-12psi of fuel pressure at the IP
inlet. On mechanical IPs, limit/regulate for 6-9psi at IP inlet, so the pressure regulated timing is not thrown off.
It is also possible to use two LPs in series to get better pressure. The OEM type LPs have a high failure rate
within 1 year. The Walbro FRB5 is a good LP for better reliability and better pressure than OEM, but even it
can be taxed by high performance programming, so it could be assisted by keeping the OEM LP in series. If
purchasing OEM LPs, select the unit for 1993 6.5TDs (Delphi HFP905, ACDelco EP158 or P/N 25115224) as
it is usually cheaper, and about 1psi higher pressure. Airtex E8153 is quite cheaper, same style as stock, but a
pressure spec of 14psi, although lower flow capacity and likely not as reliable as Walbro. While under the truck
inspect all the fuel lines for rot and rust, little pin holes before the LP will cause air in the fuel with frustrating
fishbiting, stalling, timing and IP codes. A dead LP will allow the IP to create a vacuum in the entire line for
ever greater chance of air in the fuel.
(11) PCM programming upgrades for towing, high performance. This is where a staged chip for your truck
could be convenient, for a safe option for others to use, options for fun and the ones you watch the gauges on.
(12) Cooling upgrades for towing, beneficial for reliability at any point, and an absolute must for a hard
working engine. It is important to also understand that cooling upgrades could result in loss of some efficiency.
Hotter engines can run more efficient, and cooling systems take power to remove heat. But when properly set
up, with the right thermostat(s) and the fan clutch engaging correctly you'll do well. Until 1997 a 90gpm water
pump with unintentional unbalanced flow between heads was utilized with an inefficient 6-blade steel fan. Fan
clutches also lose engagement ability in as little as 5 years, along with not engaging early enough. I highly
recommend the 2000 year 130gph OEM water pump with spin on fan clutch (ACDelco 251-603), as it has the
best flow balance to prevent #8 cylinder damage. When upgrading to this water pump you will need a spin on
fan clutch. The OEM spin on fan clutch is acceptable with the 2000 year OEM 20" 9-blade steel fan, however
the bimetal coil on the front of the clutch may need modification to engage near 210 ̊ F coolant temperature.
Sustained coolant temperatures over 220 ̊ F while under heavy load can cause engine damage. To modify the
clutch engagement remove the coil held in by a glob of adhesive/silicone and shorten the bent tab that holds it in
place. Shorten it by flattening it out and bending it 90 ̊ about 1/8" closer to the end of the coil and securing
back in place with some silicone. Then test the engagement temperature under heavy load. The fan is easily
felt and heard when it engages, and for efficiency you do not want it before 210 ̊ F. However, if this is a tow
vehicle you may choose to upgrade to an HD clutch with a plastic Duramax 21" fan (ACDelco 15-80690).
The Dmax fan fits to the OEM threaded clutch, is lighter, and pulls more air than the 9-blade steel fan. It pulls
more air even at idle, keeping engine bay temperature low at all times. HD clutches have a higher engagement;
standard clutches may engage 60-70%, where HD clutches can get 80-90% of full engine speed and last longer.
I recommend keeping the single thermostat housing, because it pushes all water to the radiator and doesn't
bypass some warm water back into the heads like the dual thermostat housing. The single thermostat housing
also gives you easier access to the IP area, making the timing procedure very simple. Those with dual
thermostats should restrict the bypass hose or fitting that goes to the water pump, to about 3/8” diameter. I’d
always use an AC Delco 195 ̊ F thermostat (12T45E for single t-stat) for best engine efficiency. Using a lower
temp t-stat could cause a loss of 1-4mpg. If you normally cruise at more than 195 ̊ F, the radiator likely needs
to be cleaned by removing the radiator, which should be done as periodic maintenance. A radiator shop can hot
tank them for best results inside and out. Or use a few applications of scrubbing bubbles and hot water, clean
with a garden hose; high pressure sprayers could damage aluminum fins and cores. Cap the tranny oil ports
when cleaning the radiator, but if also flushing it, don’t forget to add fluid later.
When refilling the system with water, consider using less than 50% anti-freeze concentration. Anti-freeze
lowers the cooling efficiency of water significantly. If you do not live in cold climates, or towing your big
summer camper, consider using only 10-20% anti-freeze and a bottle of Red Line Water Wetter or similar
product. The Water Wetter contains the lubricants and corrosion inhibiters that anti-freeze would provide, and
increases water’s surface tension to prevent localized boiling.
(13) Upgrade Glow Plugs to Bosch Duraterms, part # 80034, self regulating, quick heating GPs that have
proven the best for performance and reliability in the 6.2/6.5 diesels. AC Delco 60G GPs are also self
regulating and reliable, just take longer to heat to full temperature. PCM programming can increase GP time, so
either plug is acceptable. To check GP health, disconnect the spade terminal and with a digital multi-meter
check the ohms between the spade on the glow plug to its threaded body screwed into the head. There should
not be more than 1 ohm resistance through a cold plug, but make sure there is some, like 0.4 ohms, because an
open circuit, or “overload/OL” on a multi-meter means it has completely failed. Poor GP performance could
also result from a failing glow controller, so when changing the plugs, consider changing the controller as well.
Often times short glow cycles will be caused by failing glow plugs, because they will not draw a lot of current
(amps) from the batteries that should pull voltage down under 11 volts. Glow plug time is determined by
battery voltage, ECT and IAT when turning the key on. For max glow time, unplug both the ECT and IAT
sensors. At battery voltages over 11V the PCM begins reducing glow time significantly.
(14) Upgrade the fuel line from the FFM to the IP. There is a 1/4" fuel line and fittings to supply the high
pressure fuel injection IP, but 3/8” fuel line supplying the FFM. This modification is commonly referred to as
"Feed the Beast" (FTB) and there are kits available at WalkingJdesigns website. For those wanting to find their
own fittings it is good to know that AN (Army Navy) fittings are standard sized, just divide the dash number by
16, i.e. -6AN/16 equals 3/8, so the inner diameter (ID) is 3/8”. AN fittings have the same threading as JIC
fittings, and at these pressures are interchangeable. The FTB 2.0 Stainless kit online is probably the best way to
go. When drilling the FFM for 1/8” MPT to 3/8” fitting of choice, be careful not to hit the central fuel input
shaft; I cleaned the last edges with the bit backwards, or you may grind the bit tip flat. The lower to upper IP
fitting is an uncommon 7/16-20 O-ring interface. A full flow steel upper IP fitting is rare, but can be purchased
at WalkingJ. If routing the fuel line under the intake to the FFM, a full flow 120° to 150° 3/8” hose end should
be used to attach the line to the IP. With some hose and wire you can also relocate the FFM for easier access.
(15) Upgrade the turbocharger to a larger one, such as the T3 HX40 or HX40WII turbo chargers, which would
require some minor custom fabrication. Check out the small GM8 next to an HX40WII below. The HX40
turbine and compressor housings, and wheel diameters are larger for better flow. The HX40 has an internal
wastegate with adjustable boost referenced actuator for 20+ psi. The HX40WII is sized for fast spooling and
the simplest installation with a small footprint, but the 16cm2 HX40 turbine housing provides a better overall
match to the 6.5TD where less wastegate actuation is needed. A T3 18cm2 Holset turbine housing is also
available for the really highly fueled trucks. The larger turbine housings will cause some lag. A WII may start
spooling around 1100rpm, compared to 1400-1800rpm of the larger HX40s. The GM8 is a restriction when
increasing fuel output. The small turbo causes more parasitic engine backpressure than larger more efficient
turbos that also reduce IAT and EGT, and sustain the power band to red line. With no EGR system the vacuum
pump can be removed and a shorter serpentine belt used (aprox 101" belt for 94/95, or 99.5” for 96+). I
recommend the vacuum controller for the GM turbo, but the vacuum pump can also be removed if using a
mechanical actuator. More info on the next page.....
For the serious performance seekers, the lower IP
inlet fitting has four small holes (about 1/8”
diameter) that can be bored out with a 5/32” bit,
however this fitting is very hard metal and difficult
to drill, even with a press. I was able to bore them
but it took quite a bit of patience and a really hard,
sharp drill bit and time on the press with oil cooling
the bit and fitting. Also, in many stock FFMs there
is a corkscrew, spring-like piece inside the central
shaft that the fuel heater sticks up through which
causes additional restriction. This corkscrew piece
will slide right out and is unnecessary. While you
have the FFM apart, consider changing the O-rings.
You will notice that some FFMs have the drain
fitting as pictured to the left, but others point straight
out like the IP output, make sure not to mix them up.
I recommend a 14cm2 wastegated turbine exhaust housing for a turbo that will handle any PCM programming,
spool quickly off the line, but not choke the engine until red line, and work well over 4000ft elevation. The
HX40 is a decent option because it has several housing and wheel sizes. I recommend requesting the 10-blade
76mm major diameter (inducer) turbine, if available, although 12-blades should spool faster. Most compressor
wheel options will be alright for the 6.5TD, but bigger is not necessarily better. A large major diameter
(exducer) compressor lowers the trim value, but can help spooling if using a large turbine housing and high trim
turbine. A smaller inducer can also help spool faster with less resistance to spin and less likely to surge, but
requires more drive pressure and shaft speeds to reach higher boost or mass flow. A 16cm2 turbine housing and
larger turbine wheels reduce backpressure and loaded IATs even more for greater power over 3000rpm, without
sacrificing much low end. Unfortunately the large wastegate actuators on these turbos interfere with large air
boxes, and not all of them adjust lower than 20psi boost. To remedy both issues you can get a “spring-gate”
controller from ptwiringsolutions website, along with an oil fitting adapter and exhaust elbow. I purchased an
HX40WII and made a custom elbow and this drawing for others. This may also be made using a 2.5” elbow.
The finished product looks like this, cost me about $65 in parts and $50 for a shop to cut and weld:
Left: This is for the 14cm2 housing of the
HX40WII. The 16cm2 housing HX40 has a 3.5”
V-band flange (4” OD). The 16cm2 housing also sits
higher, so the elbow would need to be about 1”
longer with only a 10 ̊ angle for the offset.
Below: Turbo size comparison, essentially too
small, medium, large and excessive. The HE351
while cumbersome still only has a small 70mm 12-
blade turbine wheel, not worth it IMHO.
(16) Water Mist/Methanol Injection (WMI) for towing or racing to help reduce IATs and EGTs. No more than
20% methanol mixture should be used in a 6.5 diesel, more could result in damage. These diesels are not meant
to have fuel until close to TDC of the compression stroke which can produce temperature of 2000F even prior
to combustion, but the methanol will be sucked in during the intake stroke. Relatively small nozzles, such as
300cc/min or 5gph and high pressure (100psi) are recommended for best atomization, use multiple small
nozzles if more water is needed, and only high quality nozzles that won’t drip with residual pressure. 100-
120psi water pump recommended, or regulated to 120psi so you don’t have push lock leaks or bursting lines.
Inject water post turbo and with 4psi minimum boost for full volume, or use a progressive controller based on
boost starting at 2psi, or just one real small nozzle (200cc/min) at 2psi. Some windshield wash fluid is 99%
methyl alcohol and readily available, just check the bottle. It is critical not to use more than 20% concentration.
Some extremely small nozzles that make very fine mist can be used pre-turbo, but post-turbo will ensure no
compressor blade damage. I recommend WMI over an intercooler, although intercoolers can be very helpful for
hard working and high performance engines. Diesel combustion can be more efficient and smoother with warm
air if not under load. WMI only cools the air when necessary, provides additional catalyst/fuel, has cleansing
affects, and increases compression while absorbing heat in phase exchange. Because water does not compress,
I do not recommend spraying a lot of water at any boost level, two 300cc nozzles at 100psi will make a good
IAT/EGT difference. After intake, exhaust and turbo upgrades the intercooler is less necessary, although a
good way to increase air mass flow. The GM turbo cannot efficiently/safely support additional air mass flow
from an intercooler over about 10psi of boost. A high trim compressor and high flow turbine section should be
used with an intercooler.
(17) Fuel, oils, and their filters; keep your change intervals appropriate for the oil and filtering being used. I do
not recommend anything other than diesel fuel used with the electronic DS4 IP. As previously mentioned, B5
fuel is a good choice for lubricity, but I do not recommend higher concentrations of biodiesel. That doesn’t
mean it won’t work with B20 or higher, but Stanadyne also does not recommend anything over B5. The US has
different diesel fuel specifications for B5 and anything from B6 up to B20. Anything that could impact the
transparency of the fuel can negatively impact the injection system, because the OS must see through the fuel.
Biodiesel has a significantly higher cloud point, near freezing temperatures, and other vegetable oils can cloud
and solidify at even higher temps. When switching to a biodiesel blend make sure to check/replace your fuel
filter after the first couple fill-ups, and realize it contains about 8% less energy per gallon, although has higher
cetane. Before opening up the FFM, always drain about 8oz of water, sediment and fuel out the drain hose
connected to the t-valve on the t-stat housing crossover. Draining should be done every so often as well, at least
every couple months. Normal change intervals for this filter would be every 6 months or 8000 miles, but a
post-FFM fuel pressure gauge could be your determining factor for filter changes. If you have not added a
priming switch for the LP, the 94 models can simply press the slit on top of the LP fuse (on passenger firewall)
up to one of the battery junction posts next to it. In 1995 trucks, you can put the transmission in gear and turn
the ignition to Start/Crank and it will run the LP, but it won’t crank with the tranny in gear.
There are OEM type fuel filters that have metal tops and use a separate plastic filter nut to hold them down. I
use OEM type Wix #33376. Then there are larger fuel filters that the whole top is plastic and incorporates the
filter nut. Some people have had trouble getting a good seal with the larger filter types, which can lead to fuel
leaks or air intrusion. With the OEM type it is imperative that you line up the notches by correctly clocking the
filter to the FFM. It only sets down one way for proper seating. Also make sure old filter gaskets are not left on
the FFM before installing the new filter. While the filter is out, check the “last ditch screen” fuel strainer
(Stanadyne #29244) that slides down around the central FFM shaft and commonly gets pulled out and discarded
with the filter. The last ditch screen can also get clogged with sediment and gunk, just as the “tank sock” on the
fuel sending unit (FSU) in the fuel tank can. Either being clogged can result in low fuel pressure at the IP inlet
and poor performance. However, a clogged tank sock would also show low fuel pressure at the water/fuel
drain, whereas a clogged last ditch screen would help to create good pressure at the drain hose. That is because
the drain hose is pre-filter; a restriction after it will help build pressure, another reason to monitor fuel pressure
at the IP inlet.
If dropping the fuel tank, consider removing the tank sock and instead putting in an external filter before the LP,
a simple large strainer, or a 30 micron filter/water separator setup. With the added filtration the interval on the
stock filter can be increased. Fuel heaters help maintain a low diesel fuel viscosity for optimal atomization
when injecting it into the cylinder. A scanner will show fuel temp in the IP, and if it is under 110F after driving
a while then the stock heater likely isn’t working. The FFM heater is supposed to run with ignition power, so
you can test that with the filter out, if the central shaft of the FFM gets warm. Vegetable oil, while really not
recommended, absolutely has to be over 150F before using it in the injection system, and never left in the
system at shutdown, or mixed more than 5% with diesel fuel.
If using the stock oil filter and conventional 15W-40 oil I recommend changing the oil every 4000 miles, or
every six months, whichever comes first. Synthetic oils can double those intervals, but in either case make sure
to check your oil level occasionally. If you’re adding new oil frequently you could extend the interval, and you
should ensure the crankcase depression regulator (CDR) is not stuck open. The CDR (NAPA # CRB 29445) is
the large tuna can thing on the passenger valve cover connected to the air intake, it is a positive crankcase
ventilation (PCV) valve for diesels. It is supposed to limit vacuum from the turbo, so oil does not get sucked
out. You’d be better off venting the valve cover to atmosphere (like a breather, or road “draft tube”) than
operating the engine with a non-functional CDR. Its there because engine blowby makes up nearly half the
emissions from vehicles, and having a slight vacuum in the crankcase helps prevent leaking oil seals. Older
6.2L and military 6.5L diesels pull a vacuum from the oil fill tube, where it may not suck as much oil out.
For 2WD trucks with vertical oil filters you can use a 2 quart oil filter for added oil capacity and filtering. I use
the Wix #51794, available from the oil geek as well, but it has no anti-drainback, so it must be used vertically.
4WD trucks use a horizontal filter adapter that can be changed to a remote adaptor, to add filtration, capacity
and cooling. The additional capacity itself provides added cooling benefit. More filtration and capacity,
combined with synthetic oils can increase change intervals to 10,000 miles, or once per year, whichever comes
first. Synthetic oils can be used in even longer intervals, but oil analysis is recommended to verify its condition.
In cold climates I recommend synthetic 5W-40 diesel oil, Mobil1 TDT or Shell T6, for easier starting and
smoother warm-up. I am now using Shell T5 synthetic blend 10W-30 diesel oil. Synthetics provide superior
performance at high and low temperatures, and easier flow for potential efficiency gains, using lighter weights.
Amsoil even makes a full synthetic 5W-30 for pre-2007 diesel engines. Lighter oil weights may increase oil
vapors and observed blowby, with oil consumption, so it is important to check oil level occasionally. Amsoil
has heavier synthetics with superior vapor resistance that could reduce visible blowby.
Adding oil capacity and oil cooling for hard working engines is a good way to remove heat from the heads,
cylinders and turbo. In some cases this can be more effective than water cooling upgrades. The same goes for
adding capacity to your transmission and gear differential, using larger, deeper covers that also shed heat better.
If installing a deeper transmission pan and new filter, it might be a good idea to change all of the electric
solenoids in the tranny (whole kit $140 at electricaladvantage.net) and perform the Sonnax Sure Cure kit update
($86), which can make it more reliable with a new firm feel. The Sure Cure kit also requires a reaming tool, but
this addresses the 4L80E’s most common wear items. Synthetic oils can also perform well in trannys and gear
differentials in cold climates and those that get warm often and for extended periods.
(18) Head related maintenance....ARP Head Studs are a must, never use the crummy torque-to-yield (TTY)
bolts again. Lowering compression ratio (CR); I recommend dropping the CR to approximately 20:1. This
would be accomplished by using a +0.01” thickness gasket, or -0.01” compression height pistons or later model
“diamond” etched precups. If decking the block, you must also take that loss of height into consideration. A
combination of methods, or custom thicker gaskets can be used. The standard thickness head gasket is
approximately 0.045” thick. If shaving pistons, I recommend dishing, leaving about 1⁄2” ring on the outside.
This retains the ring height for durability and could aid swirl and initial pressure towards the piston center. You
can gain considerable responsiveness using the latest style pre-cups from later year models. The diamond
etched pre-combustion chamber cups have larger openings to support more power from higher fuel rates. The
94 and 95 engines had smaller mouths and smaller inner volumes as well. GM dropped the CR in later models
by using pre-cups with more internal volume. The diamond pre-cups have slightly thinner walls and larger
opening which means more volume to lower the CR. The differences can be seen below. The larger mouths
can put more combustion expansion pressure onto the piston during peak torque angles up to around 76° after
TDC. They also allow higher fuel rates to burn more effectively, where the smallest precups can limit you.
The diamond pre-cups should come in new
heads, or you may be able to source a set from
engine builders to install in your heads. There
are genuine GM/AMG pre-cups, then there are
also aftermarket ones. The aftermarket foreign
pre-cups are made from presumably inferior
metals, as they are magnetic, whereas the
genuine ones are not. The two also have internal
cup porting differences as seen to the left, two
different diamond stamped pre-cups. The center
cups with distinct internal machine lines and
rounded external mouth corners are genuine.
(19) The harmonic balancer (HB) is key to keeping your bottom end from breaking at any power level. There
is the primary HB and then the secondary HB which doubles as the crank pulley. Cracked and broken rubber
can lead to broken crankshafts and presumably more main web block cracks. Inspect these items, and if
replacing, consider the Fluidampr HB. If not for ensuring your new found power doesn’t leave you with a two
piece crank, then do it for a smoother, more comfortable ride. The Cadillac of diesel trucks, especially with the
super comfortable captain’s chairs in 94 models. The Fluidampr HB replaces only the primary HB and it costs
about 3 times more than a standard replacement. It is a peace of mind, creature comfort upgrade, but an actual
upgrade in vibration dampening. If your stock rubber engine or transmission mounts are cracked or sagging
(common on passenger side), this could also cause excess engine vibration and turbo downpipe fitment issues.
Upgrade to stronger polyurethane mounts from Energy Suspension. Reusing your existing clamshells, the poly
inserts are part number 3.1152 and the transmission mount is 3.1129.
(20) Injectors are standard 100,000 mile maintenance items; in high power, heavily loaded trucks I recommend
rebuilding at 50,000 mile intervals. I have seen burnt nozzles dribble fuel in much less mileage. These
injectors are purely mechanical; they pop at a set pressure generated by the plungers in the IP. Stock pop
pressure for non-turbo engines is 1850psi, turbo injectors are 2100psi and marine injectors are 2200psi. The
non-turbo and turbo injectors use the same nozzle (SD304), but the marine nozzle orifice is slightly
larger/longer (SD311). They all use identical injector bodies, so the nozzles and injectors are interchangeable.
Injector health is very important for a well running, reliable and efficient engine. Poor performing injectors can
go undetected very easily. Low pop can make the engine run smooth at idle and low load, but not spray well for
efficient combustion. That can lead to high EGT and poor MPG under load. Injectors can leak overnight,
because there is always pressure in the lines, about 125psi after shutdown. Fuel in the cylinders will wash away
lubrication and cause scuffing and scratching on subsequent cold startups.
Several injector rebuilders are out there, but I prefer to have mine rebuilt with known nozzles and set to specific
pop specs. A diesel injection shop should be able to change your nozzles and set the pop pressure for about
$120, perhaps they’ll let you watch. I use marine nozzles, but standard nozzles are adequate for most trucks,
and those wanting best MPG. The only nozzles I like using are German Bosch, but Bosch also produces them
in India, and there are knockoffs too, so you may have to search a bit to find the German ones. I have my
marine injector pop pressure set to 2350psi, and recommend standard injectors set to 2300psi for applications
running more than stock boost, but will be louder. Pop pressures will settle 50-100psi lower after several
weeks. Each injector should be set as close to each other as possible, with a maximum range of 50 psi, tighter
when possible. Production tolerance on eight new injectors could be as high as a 400psi range.
Consider that cylinder air pressures approach 2000psi during injection, if running 15psi of boost with a standard
CR of 21:1. When running higher fuel rates and higher boost it is advantageous to have higher pop pressures
for better atomization. Pop pressures on other types of diesels are much higher, but I would not set it over
2700psi pop pressure because the DS4 can only build so much pressure at cranking RPM, so starting cold could
get difficult and injectors overly noisy. Too high of pop pressure could negatively affect spray pattern and
reduce IP longevity. The IP can generate ~5000psi at idle, but may normally only hit pop pressures at idle,
compared to 9000psi at high fuel rates at high RPMs. Other reasons to increase pop pressure would be mods
that reduce compressed air temp and combustion efficiency, such as low CR. Marine injectors providing more
power is not a complete myth. The marine nozzle has a larger orifice that can allow more fuel to be injected
before the injection event stops with the opening of the IP’s fuel solenoid metering valve. Injection volume is a
combination of IP transfer pump pressure, which rises with RPM (30 to 125psi), time (pulse width), and plunger
displacement. With the same amount of time more fuel can flow out of a larger orifice. Marine nozzles are
most useful with PCM programs calibrated for higher fuel rates, or “turned up” mechanical pumps.
When installing the injectors there are special sockets made to prevent breaking return nipples, such as Kent-
Moore injector tool J-29873 or OTC #5060. Injector replacement kits come with copper crush washers that
must be used. Do not reuse old copper washers, use anti-seize compound on threads, and torque injectors to
55ft-lbs. Make sure everything around the injectors is clean before removing them and handle new ones with
care, as to not foul any during installation. New injectors and high pop pressures can increase engine noise.
(21) Never use the crappy stock CS-130 alternator again, the alternator in later models was changed to the
much better CS-144, so irrelevant for those trucks. At a minimum look up the Iceberg versions of the CS-130.
Standard output is 105 amps but there are 140, 160 and 170 amp versions available online. I recommend at
least the 140 amp version. When ordering, it is important to get the right size pulley, because the tachometer
runs off of the alternator speed. The outer lip diameter of the pulley is 2-5/8”, but most are sold with the 2-3/8”
outer lip diameter pullets that come on gas vehicles. Aftermarket places also try to put smaller pulleys on to
increase output at idle, but it makes it spin faster and the tachometer reads really high. The actual inner pulley
diameter that the belt rides on is 62mm. Make sure not to get a CD-130D, it is a different configuration.
Another option is to do a little modification and install the CS-144, also available in higher amp configurations.
More information about modification required will be added later.
(22) It’s all about the flow, more head related modifications. Those after the most reliable and efficient
performance possible have some further options, which if rebuilding or refreshing an engine may be a
reasonable time to implement a few very helpful modifications. To minimize your engine’s air pumping losses
there are additional upgrades beyond the larger turbocharger. The least of which is extrude honing the cast
exhaust manifolds, this process is costly ($750), but is highly beneficial. Then there is the airflow of the heads
themselves, and with shipping and extrude honing prices it is impractical. However, you can get new cast,
made in the USA, AMG pre-ported heads from Peninsular diesel. The real difference maker though, is that you
can purchase or have Peninsular mount a set of adjustable roller rockers in the heads, and they come with
performance push rods. The roller rockers are stronger and provide less friction, together with the performance
push rods results in significantly less deflection. The rockers are also higher ratio, 1.6:1 vice the stock 1.5:1
stamped steel. Combine less deflection with higher ratio and you get extended valve duration and lift for
making better and more efficient power through the entire rpm range, and extending power beyond 4000rpm.
The pre-ported heads with roller rockers will set you back about $2500, but will pay for itself in additional
MPG and new found power. Peninsular also offers the strongest new 6.5 blocks from AMG, cast in the USA,
so could be the entire repower solution.
(23) That brings us to actual repower solutions....To be continued.....Get an AMG Optimizer 6500 if you can
No-Start Diagnostics
If the truck won’t start, first and foremost are the SES and GP/WTS lights coming on? If SES is not on with the
ignition and GP/WTS doesn’t work then you likely popped the ECM fuse, or the ignition switch isn’t sending
power to the PCM. If other stuff like dash lights or the radio doesn’t work I would suspect the switch.
Otherwise, check grounds, the ones on the passenger side of the engine, back of the intake and head. Also
make sure the battery cables are secure and clean on the batteries. If everything seems to be in good order
make sure to check the FSOL fuse, because it powers the PMD and ESO/FSO solenoid. If all fuses and grounds
are good, then verify you have fuel pressure to the IP. That involves LP diagnostics, or simply verifying that
while cranking fuel is pumping to the IP. Loosen the fuel inlet hose clamp on the IP and crank. Fuel should
leak while cranking, and if it is, then tighten the clamp again while powering the LP so any air is purged. Some
LP tricks are previously covered in the recommendations above. In a rare case, the fuel return line could be
plugged, and disconnecting the return hose on the IP and routing it to a bottle to collect fuel may allow the
engine to start.
This assumes that your engine is cranking fast enough to start. Batteries should be matched, if either one or
the starter is bad it might not crank fast enough. Batteries need to be load tested and not just stabbed with a
meter to see they are over 12V with no load. Either use a load tester with each battery isolated, or monitor
voltage at the battery with a meter while cranking. If it drops below 10V then one or both of the batteries are
bad, or check the water/electrolyte level in the batteries, fill with distilled water if necessary. They should be
replaced as a pair if one is bad. This also assumes the engine is warm enough to start or the glow plugs are
working. Check the glow system out (some info available in section 13 above), or plug in the block heater for a
few hours to see if it will start that way.
If all seems well so far, then you can try unplugging the OS on top of the IP. The truck only needs one timing
source to start and run, either from the OS or the CPS, which check each other for failure. With either not
functioning properly you may experience extended cranking times to start. If it won’t start after unplugging
the OS, plug it back in and try unplugging the CPS. When one of these two fails or is unplugged it should set a
DTC even when only cranking. If it is not setting a DTC that may reveal an issue. Air in the fuel can often
emerge as random OS or CPS codes, or simply mess with injection capability when starting. If the LP is
working with pressure at the IP, then air could still be entering between the fuel tank and the LP, where the LP
pulls a vacuum. Leaks would not necessarily be observed, but an inspection for rusted and rotted fuel lines
above the fuel tank could reveal the issue. This could show up as fish-biting while driving until it worsens to
causing backup fuel mode or not starting. Sometimes when air is entering the lines before the LP, leaving the
fuel tank cap off or drilling a hole in the cap will temporarily alleviate the issues. That is because with no fuel
cap, no vacuum can build up in the tank, so it is easier to pull fuel, so less air gets sucked in.
Hopefully you have a spare PMD, since it is often a cause of a no-start condition. Swapping PMDs is usually
one of the first things I would try. Otherwise, if using an extension cable, try eliminating it to see if the
situation improves. If that does not work, check the ESO/FSO solenoid. It should click when the ignition is
turned on, because it lifts a plunger to allow fuel flow. If concerned about the ESO solenoid condition you can
simply remove the plunger. Unplug the solenoid and twist it off the IP, and pay attention not to lose the
spring under it. The plunger is held in by a snap ring, so find a snap ring pliers, small to medium sized. The
plunger will fall right out, and you can reinstall the solenoid body. Otherwise you could also just cut the
plunger head off with a bolt cutter. The ESO solenoid is not necessary in normal EFI operation because the
PCM and the PMD must have ignition power to even get power to the IP’s electronic FS. The ESO solenoid is
essential in mechanical fuel injection systems.
Below is a table of signals you can test at the PMD engine harness. You can verify the PMD is getting power,
and that the PCM is sending injection signals to the PMD. Voltage measurements are approximate, give or
take like 10%. I got the numbers out of the GM manual and/or verified on my engine with a digital multi-meter
and an oscilloscope. Take notice that the PMD outputs a modulated pulse signal to the fuel solenoid (IP) that
can be seen as alternating current (VAC). The digital multi-meter will display very close to actual voltage as
seen on the oscilloscope, because VAC is an RMS value, and they have issues calculating the RMS of a square
wave, and tend to just show the peak to peak voltage. In this case the voltage goes from 0 to 1.2VAC, 1.6VAC,
ect... The PMD output will increase in voltage with RPMs. If during crank you have a value 50% higher or
greater, it could be that the power or ground to the IP has been broken, there should be 0.4 ohms between
PMD harness sockets B and F.
B+ = battery voltage, approximately 12V with ENG off and 14V with ENG idling
PCM connectors/pins : 1st letter is color of the PCM connector, 2nd letter is row in that connector
BC/BD = 32pin Blue connector PC/PD = 32pin Pink connector PA/PB = 24pin Pink connector
PMD tests for surging or no-start......
If the engine is running look at the fuel rate and desired idle values using a scanner. If the PCM is commanding
a steady fuel rate less than 10mm3 it should never go to 3000rpm, which you can also see by the desired idle
value. If rpm is constantly going up and down, you probably have a broken IP. The PMD output can be tested
by using the following method. You will need to access the red wire in position "B" of the PMD connector with
the PMD plugged in. This can be challenging with the intake manifold installed if using an IP mounted PMD.
Straighten out a small paper clip and push the paper clip into the back of the PMD connector right alongside
the wire to pin B. Push the paper clip gently between the rubber seal and the wire. Wiggle the paper clip and
push gently until about an inch of the paper clip is no longer visible. The paper clip should now be touching
the back of the bare metal terminal connected to the wire. If you have wire piercing probes they can also be
used, or you can strip a small section of wire to clip a meter lead onto. Remember to repair the wire
insulation later though. Now, plug the PMD connection back in. Connect your volt-meter lead to the pin B
wire and the other lead to the ground on top of the IP, with the meter to measure AC volts. Do not let the
paper clip short against engine ground or risk PMD damage.
