Intermittent Engine High Idle

[Nitrofreak]

Hey guys, Been having some issues with my 94 chevy k1500 4x4, 5.7L

The truck idles REALLY high sometimes. I will start it and it will idle in the "choke" when its cold at roughly 1000rpm. it will then idle down as it warms. That's all as it should be. But I've been having problems with it beginning to go back to idling high. For example in the middle of the summer when the motor is plenty hot I will park and notice that it is idling at 1000rpm, not its usual 600 or so. Or I will leave my driveway with it idling fine and get to school and its idling at 1000rpm again.This makes it a PITA as the truck always wants to drive forward and makes the transmission shift hard into drive. I've noticed lately if I spend an hour or so driving around town it will idle high stopped at some lights, then be fine at others.

A few times it has been bad enough to give a "service engine soon light" and it develops a terrible throttle bog ( no response), it shifts extremely hard, and has no power. I shut the truck off and restarted it, and the light went away and all was normal again. this has happened twice now.

I have tried code reading it with an OBD1, and never found a code. So what could be causing the terrible idle?

truck has good plugs, fresh wires, rotor and cap. new manifolds, new Y pipe and cat.

 

[thunderstruck]

First thing I would do is check the gasket between the throttle body and intake manifold for leaks. Just take a spray bottle filled with water, windex, etc. and spray around the base of the throttle body while the truck is running. If the idle changes while you're spraying, you know you have a leak. Easy fix.

My next suspicion would be the idle air control valve. A throttle position sensor that's only working intermittently could also cause some of the problems you mentioned.