Alright, I think I'm just about ready to pull my hair out on this one, I've been at this problem since I bought this truck 2 years ago. Hopefully someone here can help me out with this one, cause I think I'm completely stumped.


When its to full operating temp, at idle it surges and will just rock the whole truck. Cold its no problem, idles fine, and as soon as you put it in gear, or turn on the A/C or anything that puts a load on the engine, it clears up and runs right. Never ever thrown a code for it and it used to be so bad that it would underrun and stall when you put it in gear. I've tried everything I can think of, so if anyone has an idea, I wanna hear it! thanks!

could be the wastegate sticking on the turbo.. on my cars.. sometimes you'll get something known as boost creep at idle.. it's usually caused by the wastegate sticking.. or a mis-matched turbo combination.. it causes the engine to surge like you state..

although.. i could be completely off base here. i'm new to diesels..

-- WheatKing

Tried it, unplugged the solenoid and the wastegate, no change, stock GM-4 turbo, the only effect it should have on the engine is a slight difference in backpressure, not enough to make any difference, they are all fully closed at idle.

I think I've really tried EVERYTHING in the engine compartment, crank sensor, IP (several tries), injectors (3 sets), timing chain, alternator, batteries, added grounds, cyl heads, rocker arms, harmonic balancer, you name it....so I guess I'm going to try and find a '95 HD PCM. I think a HD will solve the problem, whatever the cause, because it only happens when the PCM sets a desired idle of 562! Which is ridiculously slow, and I'm sure the HD's don't idle that slow. But if anyone has another idea, I'd love to hear it!

Gasoline engine rpm will increase from air leak into plenum on cylinder side of throttle plate, similar to pulling PCV or opening vacuum line.


Fast idle motor is a 'metered' leak.


Air leak won't do it on a Diesel - no throttle plate, no intake vacuum.


Takes fuel to surge.


A few tings to check -


Try hooking up the fast idle input - ties to +12v - and see how that affects the surge.


Fuel surge will make it do that, whether from air in the lines (should get Cylinder Balance codes, store only as history DTC's, no SES), or intermittent electrical connection.


PCM failure to 'lock-on' to engine rpm causes wild attempts to balance idle load between cylinders.


Idle vibration can cause jitter from TPS, indicating need for increased fuel.


Idle vibration can affect intermittent grounds and connections, where PCM thinks it is getting a valid input and reacts.





You were posting while I was typing, so, yeah, if you started with a 1500 truck, some systems can cause grief if modified or eliminated - EGR and idle, for instance.


Try the hi idle connection - only works in Park and Neutral - any operator input will cancel it. Edited by: gmctd

When the thermostat opens try accelerating slowly to say 1500rpm's with no load in park. Hold the rpm's for a couple of seconds and watch the tach. If the rpm's go up slowly the PMD is failing. When this fails it will never set an engine code.


I had two trucks fail this way at work. One would run away, the other had cruise control with no cruise control installed on the truck.

A 95 ECM would eliminate the need for three vac wg controls into one. I'll tell you one my truck does. Sometimes when the engine's warm and you restart it. The truck idles at 750-800 RPM after it first starts. It will continue to do this till you get up to 2200+ rpm on the road and then it idles normal at 600. Go figure.