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Chevy C1500 Has An Idle Problem

Q. I have a problem with the idle, stalling while in gear. Things that I have already checked and or replaced follows: new complete distributor, plugs, wires, coil, O2 sensor, IAC valve. I have checked the compression and the high is 175, and low is 170. I had to remove the catalytic converter due to exhaust restriction.

Chevy C1500 Has An Idle Problem

There are no codes displayed or stored. Timing is correct. Vacuum lines are fine with no leaks. EGR system is operating like it should. I removed the TBI unit and removed all electrical pieces and cleaned the unit and all of the passages inside it. There was quite a bit of gunk inside the PCV passage, but it is free and clear now. PCV valve is also fine. Fuel pressure is within the specs at 12 psi.

If the truck is cold and in the PARK position everything seems fine until it reaches full operating temperature. When the truck is in gear and allowed to idle for at least a few minutes, three to five, it seems to gradually start to drop RPMs and eventually stalls out. This happens either with or without the AC being on. If I feather the gas while stopped in gear I can keep it from stalling out.

I use the factory service manuals from Helm. I checked all the ECM grounds and they are okay. I checked the wiring to the IAC valve and to my best ability I haven't found anything wrong with it. If I unplug the IAC valve turn ignition on I get voltage from the test light at all four terminals. The TPS and MAP is also operating correctly according to my scan tool.

Now when I removed the old IAC valve, there was a shiny area on the pintle. It kind of seems that is was dragging inside the bore? I inspected the bore and can't find anywhere that it would be rubbing in there. Either way the IAC moves in and out like it should.

Before I started this "tuneup" I had a leaking injector causing a rich fuel mixture and I drove it like this for quite sometime. Do you think that there could be an excess amount of carbon buildup inside the combustion chamber? How about some sort of valve train problem? Hooking up a vacuum gauge to the intake at idle in PARK the gauge vibrates at a very fast rate constantly between 19" and 20". Not sure what this means. The ignition system is working like it should.

I am at a loss right now since I don't know what else to check for. This is a very aggravating problem. Any help would be appreciated.

  • 1988 Chevy C1500
  • 5.0 liter TBI VIN "H"
  • Automatic transmission
  • 215,000 miles



The vacuum gauge reading may be a clue! Possible a valve not seating? Sticking? Or maybe even a lifter is too tight? These old Chevy trucks used to have problems with the distributors losing ground inside the housing. You need to hook up your scanner and watch the data as it acts up to see if anything is out of range.
Engine Testing With A Vacuum Gauge

Additional Information provided courtesy of AllDATA

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