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1995 GMC 3500 Pickup:
Diagnostic Trouble Codes 36, 62, 65, 78 and 99

Alex Steele
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I have a 1995 GMC one ton pickup with the 6.5-liter Diesel. I've been having all kinds of oddball problems, and recently had a garage check it for trouble codes. They came up with a 36, 62, 65, 78, and a 99. What do these codes mean, and what could the problem be?




The basic rule of thumb when dealing with multiple trouble codes stored in the Power Control Module's (PCM) memory is to clear the codes and see which return first. This tells us which codes represent the "hard failures" that may have instigated the others. Today's computers separate the "current" and "history" codes. This can indicate which failures are happening right now, and what failures occurred previously.


Trouble code 36 represents an "injection pulse width error". The injection pulse width is the "on-time" or the amount of time (measured in microseconds) that the "fuel solenoid" located on the electronic injection pump is open and allowing high pressure fuel delivery to each cylinder. A possible cause of a code 36 is a weak fuel solenoid, but a defective PCM or a poor electrical connection in a number of locations could also be at fault.


The fuel solenoid is non-serviceable and comes as part of a complete electronic injection pump replacement. Remember, we said this is a possible cause to the problem, not a definite fix. The system needs to be gone over by a qualified technician with the proper tools in order to be accurately diagnosed.


We had a related question pertaining to a Chevy 2500 Diesel pickup. This reader's truck appeared to be suffering from an intermittent failure of the "Optical or Pump Cam/Fuel Temperature Sensor" which also requires the complete replacement of the electronic injection pump.


Turbo boost sensor low, Code 65 is Accelerator Pedal Position No.3 fault, Code 78 is Wastegate solenoid fault, and Code 99 is Accelerator Pedal Position No.2 fault.


The Accelerator Pedal Position Sensors (APP) have problems along with the electronic injection pumps. This most certainly could be a multiple failure situation.

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18 Jun 2019, 11:48
RE:Ed Rowley, The melted battery post is probably caused by the constant and excessive starting attempts, the starter draws a lot of amps and probably melted the post on the battery.

A scan tool is needed not a code reader. A code reader will only work on vehicles from 1996 to present. A scan tool can read important information like injector pulse width and PCM inputs and outputs to help you determine the area where the problem may lie.
Not being there and seeing the problem is difficult on my end to tell you what to do if I don't have important information from the scan tool and other measurements like fuel pressure and electrical inputs at the fuel shut off and fuel solenoid and injector pump and so on. The problem may even be a possibly a faulty PCM.

Using a test light on electrical components like the injector pump fuel solenoid can help determine if it is losing power from its power source or have another issue somewhere else that may be cutting power ( if this is the issue). A code 36 is Injection Pulse Width Error (Response Time Long) I will list more information on this below.

The fuel injection driver receives an inject command signal from the Powertrain Control Module (PCM) and provides a current regulated output to the fuel solenoid that controls injection. It also returns an injection pulse width signal back to the PCM to inform it when the fuel solenoid has actually seated. This injection pulse width signal is measured in micro seconds.

Battery voltage greater than 10 volts.
Coolant temperature greater than 20°C (68°F).
Injection pulse width greater than 2.5 milliseconds.
ACTION TAKEN (PCM will default to)
Fixed injection pulse width value of 1.95 milliseconds.

The fault condition(s) no longer exist, and the ignition switch is cycled "OFF" then "ON."

Number(s) below refer to circled number(s) on the diagnostic chart.
This step determines if DTC 36 is a hard failure or an intermittent.
This step sill determine if the solenoid is at fault, or if there is a problem with the PCM or wiring.
The injection pulse width will fluctuate when throttle is depressed.
A weak (mechanical failure) fuel solenoid will result in DTC 36.

I hope some of this information is helpful.

Information is from Alldata automotive information website.

Ed Rowley
18 Jun 2019, 10:48
Hi again, we didn't measure the fuel pressure but it does come out the hose full bore so figure it has good pressure. It runs fine when it runs, it just won't start. Oh and I forgot to mention that after we got it back the last time one of the new batteries had one of the side posts melted almost off.
Ed Rowley
18 Jun 2019, 10:34
Hi TechHelp2: We have been taking it to a (what we thought was a) qualified mechanic with testing equipment. Thousands of dollars later we are still having problems, can't afford to put much more money into this "95". In addition to all the other things I listen earlier, it's had a new starter, then a bolt broke off on the starter, a couple months later, so took it to another garage, he said the grounds were bad, one in the starter and on the truck and the battery terminals were bad, batteries were border-line. So replaced both batteries. The 1st fuel shut-off was loose so put another one of them on and a new fuel filter. It hadn't run since we got it home from the starter fix, so when it did start one day (surprise) took it back up to old mechanic, it quit 3 times on the way there but did start again. When he went to put it in garage to work on it, it wouldn't start. He changed the oil sensor again and the fuel filter. We picked it up drove it several times maybe 50 km, was turning it around in the yard one day, it quit like you turned the key off. It hasn't started since. Now both new batteries are stone dead. When we got it back you now have to shut the heater fan off it doesn't work on the key anymore. It did start and work for awhile after he put the new glow plugs in but we're not sure what kind he used, afraid to ask, not sure he isn't part of our problem. We've been trying to buy our own code reader so we can test it ourselves but haven't been able to find one that will work on a diesel that old.
18 May 2019, 08:58
RE: Ed Rowley, Did you check the fuel filter, fuel contamination, air in fuel system and or quality of the fuel and also the fuel pressure?

You mentioned you put in glow plugs, there is a difference in glow plugs with reference to low heat energy units. These plugs can be identified by looking at the number on the plug and see if there are slashes (///) in the number on the base of the plug not the hex. This could contribute to hard start and poor running cold. This is from Service Bulletin No.: 00-06-04-002.

The system needs to be gone over by a qualified technician with the proper tools in order to be accurately diagnosed if these suggestions don't work.
Ed Rowley
17 May 2019, 15:04
I have a 1995 GMC 6.5L diesel it's only a 1500, but it's called a Heavy Halfton.
Can't seem to keep it running, have put new PMD with kit to get it off the motor, gas pedal sensor, fuel shut-off sensor(2), oil pressure sensor, new rad, new turbo, new glow plugs, quit again and won't start.
mike curtis
02 Aug 2014, 11:25
I have a 1995 gmc 3/4 2 wheel drive. It has a code 36. I have a new PCM, and fuel pump. What else can I do to find out what's wrong? Sometimes it runs good and sometimes not.
15 Oct 2012, 10:03
Being your first diesel I suggest getting it to an experienced diesel tech.

First thing to check is air in the system (needs to be bleed following a filter change). Then look for water or other contaminants in the fuel.

After that it's more involved testing of the pressure and injector pump.
24 Sep 2012, 21:33
Bought a '95 Chevy diesel 4x4 for my driveway plowing. It starts, runs for 3 minutes then stops. Changed oil pressure switch, new fuel filter, fresh diesel, still starts and runs for maybe 3 or 4 minutes. I did drive down the road a little ways and back, and it ran good. I guess I got lucky because when I got back it stopped. First diesel, always gas before. Any thoughts? Thanks
25 Mar 2012, 14:20
P0446: EVAP Vent System Performance
P0452: Fuel Tank Pressure Sensor Low Circuit Voltage

Very likely the 2 codes are related -- both EVAP.

First thing to check is the FTP sensor and all related circuits.
Dean Mihalovic
21 Mar 2012, 18:49
i have a 99 gmc c2500 2wd with a 5.7L vortec. i have a code reader but cant find out what the codes mean the codes are PO446 and PO452.
15 Jan 2012, 09:21
I love reading these articles because they're short but informative.
27 Jul 2010, 20:24
I suggest you bring it back to the technician who did the gasket (not sure what gasket you're referring to?).

If that's not possible, you're gonna have to get the schematic and/or installation instructions from the cooler manufacturer in order to hook it back up correctly.
26 Jul 2010, 13:50
hi just had the gasket changed on my 1995 GMC sierra 6.5 litre diesel the wires are now missing to the FSD cooler i had installed 1 year ago. I think the mechanic plugged the wires back into the old injection pump. Where is this and how do i safely remove wires?


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