Diesel RPM surge

Tiny
JJG_30
  • 1999 CHEVROLET SILVERADO
  • V8
  • 4WD
  • AUTOMATIC
  • 120,000 MILES

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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Monday, January 10th, 2011 AT 6:18 PM

24 Replies

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

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Monday, January 10th, 2011 AT 6:48 PM
Tiny
JJG_30
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The problem occurs both at idle or while running ie under cruise control.

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Monday, January 10th, 2011 AT 7:27 PM
Tiny
DR. HAGERTY
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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.

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Monday, January 10th, 2011 AT 9:34 PM
Tiny
JJG_30
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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?

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Tuesday, January 11th, 2011 AT 4:16 AM
Tiny
DR. HAGERTY
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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

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Tuesday, January 11th, 2011 AT 5:50 AM
Tiny
JJG_30
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YEL/BLK ---XXXX--- GRY

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Tuesday, January 11th, 2011 AT 9:44 PM
Tiny
JJG_30
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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.

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Tuesday, January 11th, 2011 AT 9:53 PM
Tiny
JJG_30
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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.

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Wednesday, January 12th, 2011 AT 4:03 PM
Tiny
JJG_30
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Dr. Hagerty
Any luck yet on finding the specs for the Accelerator Pedal Position Module?

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Saturday, January 15th, 2011 AT 11:55 PM
Tiny
DR LOOT
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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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Monday, January 24th, 2011 AT 5:38 AM
Tiny
JJG_30
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I'm not sure I understand the mode of failure with the stepper motor you described. Are you saying the injection timing stepper motor is "up stepping" momentarily then correcting itself by "down stepping" to its normal position without any pulses from the PCM? Also would a change in timing cause such a high variation in RPM without an increase of injected fuel volume?

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Monday, January 24th, 2011 AT 4:31 PM
Tiny
DR LOOT
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You will need to have a scanner to verify exactly what the timing is at before start up and during idling if you do not have access to a scanner it is going to be very difficult to determine the variation in timing.

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Monday, January 24th, 2011 AT 4:48 PM
Tiny
JJG_30
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I have a scanner and had it connected for some timewhile driving and idling without an occurrence of the problem. The other day while leaving a parking lot and starting the engine the momentary high surge in engine RPM occurred(no scanner attached at the time). By high RPM I mean a very short duration of approximate 1500 to 2000 RPM; like depressing the accelerator pedal. It seems like the occurrence of the problem is considerably less in cold weather.

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Tuesday, January 25th, 2011 AT 12:04 AM
Tiny
DR LOOT
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Hook your scanner back up go to the timing section and tell me where the timing is set at, with the vehicle not running, and with the vehicle running.

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Friday, February 4th, 2011 AT 5:52 PM
Tiny
JJG_30
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Dr. Loot
I'm not sure how to get the timing info you suggested. I hooked up my scanner engine off key on but did not get any timing info or didn't know how to access it. Any help on how to access it would be greatly appreciated.

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Saturday, February 5th, 2011 AT 4:18 PM
Tiny
DR LOOT
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I would love to help you fix it I have done to many of these, you will a real scanner like a snap on, maybe you have a friend that has one?

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Saturday, February 5th, 2011 AT 9:23 PM
Tiny
JJG_30
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Thanks, I'll have to ask around. Meanwhile what are your expectations relating to timing in my situation?

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Sunday, February 6th, 2011 AT 1:20 AM
Tiny
DR LOOT
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All I know is that's where you start, when you have the symptoms you are describing.

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Sunday, February 6th, 2011 AT 5:35 PM
Tiny
JJG_30
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Do you know if Auto Zone or Advanced auto parts would have the more sophisticated scanner to get the timing data? Also having limited experience with diesels, it's hard for me to understand how timing variations without additional injection volume could give such a large difference in RPM. I thought that injection advance was relative to increased RPM due to latency fuel burn time. However if you have experienced the problem, I'm all ears.
Thanks

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Monday, February 7th, 2011 AT 1:25 AM
Tiny
DR LOOT
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For example a gas engine if you twist distributor advancing the engine, the engine RPM will increase. On your specific application the timing is control by the computer, which uses the stepper motor to control the timing, if the computer loses its parameters within the timing specifications, the engine will idle and run erratic

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Monday, February 7th, 2011 AT 1:44 AM

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