DIESEL RPM SURGE

1999 Chevrolet Silverado

Tiny

JJG_30

January, 10, 2011 AT 6:18 PM

What is causing a sporadic engine RPM surge without depressing the accelerator pedal on a 1999 Silverado 6.5L turbodiesel?

The problem has occurred at idle in park with no one in the vehicle; while in cruise control on the highway; after refueling and pulling out of a gas station and several other occasions while moving.
No DTC codes or freeze data was stored in the OBD. Approximately 2 yrs. Ago a new injector pump and PMD was installed by a GM dealership. The PMD was remoted from the pump. The problem is about a month old.
I removed the accelerator module and measured the resistances of the three potentiaometers supposedly contained in it but the values didn't make too much sense. I couldn't locate a spec sheet for the accelerator control module. Ever hear of such a problem?

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Tiny

Dr. Hagerty

January, 10, 2011 AT 6:48 PM

Idle surge in a diesel could be due to advanced timing.

Tiny

JJG_30

January, 10, 2011 AT 7:27 PM

The problem occurs both at idle or while running ie under cruise control.

Tiny

Dr. Hagerty

January, 10, 2011 AT 9:34 PM

The last repairs performed by or at the dealer were not the issue I assume. This just recently began and no faults are set as far as I can tell from here. I would next look at live data and see what the parameters can tell you, start here.

Tiny

JJG_30

January, 11, 2011 AT 4:16 AM

Correct, the repairs performed by the dealer do not relate to the current problem. The problem which initiated the repairs was an occasional shutting down of the engine.

I've run the truck with the scanner on but the problem never came up during these periods. Apparently the PCM is interpreting the anomaly as a valid accelerator pedal depression. What live data parameters would I look for, I would guess RPM's would definitely increase.
Does anyone know what the three potentiometers shown in the Accelerator Pedal Module (gas pedal) should read?

Tiny

Dr. Hagerty

January, 11, 2011 AT 5:50 AM

2 of the pots should read opposite of each other for plausibiity, the third is for overall circuit monitoring. A 5volt reference is applied to the first 2 so that when the 2 outputs are compared, they always add up to 5volts. I wil research the actual values and get back to you straightaway

Tiny

JJG_30

January, 11, 2011 AT 9:44 PM

YEL/BLK ---XXXX--- GRY

Tiny

JJG_30

January, 11, 2011 AT 9:53 PM

I accidently hit enter on the last reply before I completed what I intended to send. I was attempting to draw the POTS and describe the resistance readings. I'll use a graphics program to generate it and send it to you later.

Tiny

JJG_30

January, 12, 2011 AT 4:03 PM

Attached is the resistance measurements made on the Accelerator Pedal Module. Unless there is more than a conventional potentiometer in the module, some of the reading are not understandable.

Tiny

JJG_30

January, 15, 2011 AT 11:55 PM

Dr. Hagerty
Any luck yet on finding the specs for the Accelerator Pedal Position Module?

Tiny

Dr Loot

January, 24, 2011 AT 5:38 AM

Dr. Hagerty first response was correct "Idle surge in a diesel could be due to advanced timing." You have a bad stepper motor (controls the injection advance) on the injection pump located on the left front, don't worry dear item only $500

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