I have a 1990 c/k 1500 2wheel drive silverado with a 5.7 tbi motor and five speed trans. The truck idles high then low then high etc. it does this at all engine temps. I already replaced the engine temp.sensor with an aftermarket oreilly part, this did not help. I also tried swapping the electronic control module with one from a good running vehicle, and even tried swapping main computer box from a buddys truck. The fuel pump sounds healthy as far as I can tell, and spray from injectors looks good. I put in new plugs and cap right after replacing head gasket. so far the only way I have found to change this behavior, is to unplug the temperature sensor once the truck is warmed up. It runs excellent with the sensor unplugged. The problem with doing this is it will not stay running after cold starting it with the sensor unplugged. I really like this little stepside, but it is driving me crazy trying to figure it out. any suggestions would be greatly appreciated.

 

Might try replacing the idle speed control motor on the side of the throttle body. The plunger should move fairly freely in and out. Check out a junk yard theyre cheap. You can find em on suburbans trucks and astrovans.