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

Alex Steele
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seen in Truck Trend



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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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?
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.
15 Jan 2012, 09:21
I love reading these articles because they're short but informative.
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.
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.
24 Sep 2012, 21:33
Bought 95 chev diesel 4x4 for my driveway plowing. It starts ,runs for 3mins then stops. Changed oil pressure switch, new fuel filter, freash diesel,still starts runs for maybe 3or4 mins. did drive down the roada little ways and back and it ran good I guess I got lucky because when i got back it stoped. first diesel allways gas before.Any thoughts thanks
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.
mike curtis
02 Aug 2014, 11:25
I have a 19995 gmc 3/4 2 wheel drive it has a code 36 pwm I have a new pcm pump pmd what esl can I do to find out what worng some time it runs good and some time not


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