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Engine surge

Tiny
JJG_30
  • MEMBER
  • 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

28 Replies

Tiny
DR LOOT
  • MECHANIC
  • 2,311 POSTS
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
  • MECHANIC
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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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  • 11 POSTS
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
  • MECHANIC
  • 2,311 POSTS
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
  • MEMBER
  • 11 POSTS
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
  • MECHANIC
  • 2,311 POSTS
I would love to help you fix it I have done to many of these, do you have a scanner?
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Saturday, February 5th, 2011 AT 9:23 PM
Tiny
JJG_30
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  • 11 POSTS
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
  • MEMBER
  • 11 POSTS
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
  • MECHANIC
  • 2,311 POSTS
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
Tiny
JJG_30
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  • 11 POSTS
When you say erratic, are you saying a significant amount of RPM change?
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Monday, February 7th, 2011 AT 3:09 AM
Tiny
DR LOOT
  • MECHANIC
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Yes, up and down
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Monday, February 7th, 2011 AT 3:26 AM
Tiny
JJG_30
  • MEMBER
  • 11 POSTS
I've timed many a gas engine and I'm aware of the RPM changes as you vary the timing however in the case of my diesel, it's more like someone momentarily depressed the accelerator pedal. Suprisingly, I have not had a single occurrence in the past few weeks or more. Maybe it's temperature related. It's been cold.
Still trying to locate someone with a Snap-on scanner.
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Monday, February 7th, 2011 AT 7:34 PM
Tiny
DR LOOT
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Send me your e-mail address I need to send you some photos that this site will not allow to download, wrong program
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Monday, February 7th, 2011 AT 8:55 PM
Tiny
DARRINK91
  • MEMBER
  • 1 POST
  • 1999 CHEVROLET SILVERADO
  • V8
  • 4WD
  • AUTOMATIC
  • 141,000 MILES
I recently was running codes for bank 1, 2 running lean and my engine started acting up it would surge. It sounded like it was going to shut off. My engine light went off and the truck is still doing it. I have the 5.3 v8. My question is what could it be that would be causing this? It is getting worse now. Could it be fuel pump? Cracked intake manifold? Intake manifold gasket? Please give me some input thanks
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Wednesday, December 4th, 2019 AT 1:55 PM (Merged)
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,755 POSTS
If you have a lean mixture, it could be many things. I would start by looking for vacuum leaks. Check all hoses and the intake for leaks.

Let me know what you find.

Joe
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-1
Wednesday, December 4th, 2019 AT 1:55 PM (Merged)
Tiny
MAEMILE
  • MEMBER
  • 1 POST
  • 1999 CHEVROLET SILVERADO
  • V8
  • 2WD
  • AUTOMATIC
  • 100,000 MILES
I work at a lg chevy dlr and therefore feel weird as oking for help here but they have been chasing this problem for a month and it is costing me a fortune in guesses. They just repl my intake man gskt due to leaking and overheating. Since then my truck will surge and buck and has died on 2 occassions. It only happens sometimes. We have replaced plugs, wires, cap and rotor. Yesterday while acting up it set the ck eng lite and the code was mass air flow sensor which we replaced and still messed up. Is there something that could have been loosened or a wire pinched when the intake was done?
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Wednesday, December 4th, 2019 AT 1:55 PM (Merged)
Tiny
WRENCHTECH
  • MECHANIC
  • 20,761 POSTS
This may sound foolish but have they check it over carefully for any intake snorkels or breather tubes that may be left off or leaking? Have you tried cleaning the MAF.
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Wednesday, December 4th, 2019 AT 1:55 PM (Merged)
Tiny
02REDCAMARO
  • MECHANIC
  • 65 POSTS
I would try replacing the coil, or coils. I had a similar problem with an '02 V6 camaro. The cat converter went bad, causing engine and underhood temps to rise. Once warned up, the engine misfired, hesitated like you wouldn't believe. I replaced the MAF, plugs, wires, filters, and fluids. After I replaced the converter, I still had the problem. Problem went away after I replaced 2 out of the 3 ignition coils. The computer won't tell you if the coils are bad, no trouble codes for that, only 'misfire'. Could be ignition module, but I would go for that last. Coils can be cheap on ebay, that's where I got mine. Also check all wiring to the ignition module and coil(s). Also check crank trigger sensor, especially if it is the type that bolts onto the frame and it is a rod that points to the harmonic balancer. If it is damaged, your ignition system is junk.
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Wednesday, December 4th, 2019 AT 1:55 PM (Merged)

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