Accelerates by itself, no throttle input.

[ginger743]

I have a 1996 Silverado 6.5 , C2500 with a 4 Speed Electronic Automatic Transmission. It has 105,000 Miles on it. 5,000 since the Engine was remanufactured

I'm the second owner of this truck and it has spent it's entire life in the Desert, Las Vegas so corrosion isn't too much of a problem :>) It's been well cared for. The Engine was remanufactured because it " exploded " , I think it went out of time , ( got too far advanced ) and destroyed the heads so I had it totally remanufactured.

The Engine was remanufactured about 5K miles ago and everything was great untill one afternoon last July I was driving down the road at approx 45 MPH and the Truck accelerated all by itself, I put on the brake and pulled off to the side of the road as soon as possible and shut it off. I waited a few minutes and started it and drove away ok but it will intermittently accelerate all by itself. When it does this it puts out heavy black smoke like whatever is causing this is apparently commanding it to give it a heavy shot of fuel. There are no " check Engine Lights " or any other warning lights. When it runs without this acceleration it runs normally.

I haven't been driving it but I'm getting ready to take it to a dealer the 1st of the year.

I'm just guessing but I think it's the PCM because it does this so fast leading me to believe it's electronic rather than mechanical. It accelerates when there is no throttle pedal movement , just running a steady speed. The Injector pump wasn't replaced at overhaul but it was checked and found to be ok.

I did a search for something simular here but didn't come up with anything so I started this thread.

I sure hope someone can help me with ideas and info before I put myself at the mercy of the dealer.

I'm not taking it to the shop that remanufactured the Engine because he's mostly a machine shop. Not too much into " electrical stuff " .

Thanks for any help or ideas you can give me , Jerry

[john8662]

Hi Jerry,

It's indeed in the fuel system.

The Pump Mounted Driver comes to mind as a first suspect. Search for PMD, or FSD (Fuel Solenoid Driver) to get all the info on these devices.

I've heard of the aftermarket PMD's having issues in this department. The injection pump itself can fail (not common at all) and do this same thing.

See if you can identify what PMD is on the engine now, An OEM part is a
"Stanadyne" black box on the side of the pump.

John

[ginger743]

Hi John,

I should have given this history , sorry about that.

Before the engine " exploded " it was running great , one day I was in the desert and it acted like it was running out of fuel and it died , and I couldn't get it started after that. I checked the lift pump and it was ok. I had it towed to my home and I replaced the PMD with an external mounted one. It started after the PMD replacement but it sounded like it was destroying itself and started to smoke so bad , not black smoke, I had to shut it off before someone called the fire dept. On teardown both heads had cracks in the combustion chambers. ( It in fact had destroyed itself ) :>) The same replacement externally mounted PMD is still on the vehicle and it ran 5,000 miles ok after the engine remanufacture with that external mounted PMD that I had put on before the Engine blew up. I have a new PMD but I'm afraid to put it on after the " blow up " experience, that was super costly.

I'm tempted to try the new PMD that I have but I'm afraid to.


Sorry I didn't give this history before .

Jerry

[Robyn]

Hi Jerry

There is no connection between the PMD and the engine going away.

What is likely happening is that the PMD is failing.

These little magic boxes can fail in one of two ways, its either a stall and no start when hot or the other failure mode is what your seeing, the black smoke and engine run away.

I would swap out the PMD for a fresh one thats new or known good one.

The cracked head thing is something that can and does happen on these engines.

Change the PMD and let us know.

If after you change the PMD, the same thing happens again, then the cause is likely to be the Injection pump is going south.

Has there been any codes??? (SES Light coming on)

Let us know.

Missy

[JohnC]

There may be codes even though the CEL did not come on. Have them checked.

Hook a fuel pressure gauge up to the water drain valve and watch for fluctuations in pressure. When was the last time the fuel filter was changed?

Then PMD can fail in a manner that results in uncontrolled fueling. Also, the fuel solenoid can stick and also cause uncontrolled fueling. If the new PMD doesn't cure the problem, try a heavy dose of a good fuel conditioner, like FPPF, to see if you can "unstick" the solenoid. Also, it you can get a scanner hooked up and monitor the fuel solenoid closure time, that may shed some light on things.

[Robyn]

X2

The fuel solenoid deep inside the injection pump is in constant or near constant motion.

The ULSD fuel that we have been saddled with lacks lubricity that was found in the earlier fuels.

This issue combined with the normal amounts of dirt that finds its way into the fuel system can and does wear the moving parts in the fuel solenoid.

If the solenoid starts to Gall and bind, all sorts of hinky stuff can occur.

The uincontolled acceleration can becaused by a bad circuit in the PMD.

"USUALLY" when this happens the thing will not "Heal itself" but will, if started, just run wild until the ECM detects overspeed and shut the thing off with the electic shut off on top of the IP (Little round gizmo)

I have not seen an on again off again overspeed, usually once it happens thats it, the thing will go to full fuel when started and scream until the Auto shutdown sequence is activated.

The shut off solenoid is actually not normally used to stop the engine, but instead the loss of power to the system shuts the main fuel solenoid completely.

If for instance the fuel solenoid were to jam in the WOT position, there would be no way to stop the engine, it would go wild until it came apart.

A failed PMD can make things exciting for a few moments. The engine RPM will hit about 4000 real quick, with huge volumes of BLACK smoke, folled by the engine shutting down abruptly.

Cranking the engine at that point will not produce any results.
A Key off/Key on cycle will reset things and the process will repeat with a howling engine, smoke and an abrupt shut down.

I have only seen a few of these types of failures.

Had one myself when testing a handfull of used PMDs.
Very exciting :eek:

Another one was with a friends 6.5 and it happened in his driveway when his daughter fired the rig off to go get PIZZA.

The last time was in a parking lot and a late 90's Burb was howling wilding with the rear tires screaming in protest as the rig brodied around the lot.

I ran over and screamed at the driver to shut it off.

As long as the engine can not reach RPM limits the ecm will not shut it off.

The poor soul in that Burb was terrified.

After they finally got the beast shut down, I explained to them what was wrong and they had it towed to a shop locally and got a fresh PMD installed.

Most shops have never seen one of these go Wild and don't have a clue.

The GM shops will want to install a new IP but thats usually a big waste of the customers $$$$ as all thats wrong is the PMD is bad.


As John mentioned, getting a scanner on the thing and seeing whats going on is a great idea.

Usually the PMD itself will not toss any codes.

The exception is when you are rolling down the road at speed and the PMD fails.
This condition will leave the engine still rolling along and the ECM will get Hinky info from the fuel solenoid.
When this happens the SES light can come on.

If your at slow speed and the engine just dies, there is in many cases not enough time for a code to set.

Just rest assured that replacing the PMD is not going to cause your engine to grenade.

Be sure to swap the resistor thats in the plug cavity on the PMD to the new one.

These can be coaked out of the cavity with a paper clip end that has a little BUR formed on the end.

Slip the PC end into the holes in the resistor and alternately pull out until the resitor slides out on the pins.

The resistor is used to do the final calibration of the IP/ECM interface.

The resistors come in values from 1-9

If you do not know what should be in there, a #5 will usually keep most systems happy.

Missy

[ginger743]

Hi Missy and John,

I replacee the PMD and the problem is solved. I drove it apprx 20 miles this afternoon and it runs great , no problem.

I didn't drive it very far when it failed last July, I just drove it home and parked it.

Missy I verified that the resistor was a 5.

John , I had all of the filters replaced at overhaul. My plan was to take it to the dealer and have the code read if there was one in case this didn't fix it. I don't have access to a reader.

Missy , I really enjoyed your description of the problems that you have seen or experienced I was chuckling as I read it ;) When it happened to me I was on a 6 lane divided boulevard and I was in the middle lane coming up behind a car at a stop light, that's when it accelerated, I couldn't get off to the side of the road quick enough .:D After I stopped on the side of the road I said to myself, " this thing is gonna kill someone " :confused:

This web site is a fantastic resource !!! I notice how many posts you and John have posted Missy and how long you both have been members, WOW , what a great resource we have here !!

I can't thank both of you enough for the help. You can't imagine how this has bugged me since July. I love that Truck and really missed it.

Again, thank you so much John and Missy, you have helped me way beyond my expectation !!!

Thanks again and have a great New Year , Jerry

[JohnC]

Glad you got it fixed, and thanks for the kind words!

[donut_1995]

i know this is an old thread, but i have encountered the same problem but with no results from changing the pmd. I guess my point is, are there any components in the IP that can be easily changed that would cause this problem if it is in fact the IP. I can't afford a new pump right now and would greatly appreciate any input!
Thanks for any information!