Seems to be plaguing many C4 owners. The problem is that there are so many things that could be wrong.
I would suggest a couple things:
Consider cleaning the throttle body. (about 1.99 for throttle body cleaner, don't use carburator cleaner). Also on the throttle body is the IAC sensor, it is normally cleaned/replaced when the throttle body is cleaned since it mounts in the side. Its a very sensitive unit and its has threadlock on it, so after getting it broke free its easy to unscrew. Trouble many have is getting a socket to fit it, best advise I've heard is that a wrench can be fit on it to break it loose. There are many cautions about cleaning the IAC, usually dipping it in the cleaner and brushing it with an old toothbrush, don't touch or press on the tip. Make sure the eyelet gets cleaned. Its best to remove the throttle body. Some have been successful at removing the throttle body but not removing the coolant lines, I haven't done that yet but it would save a big pain of draining and refilling the coolant. (Unless yours needs a refresh of fluid). Be careful not to damage the gasket and it can be reused.
Check the plug wires. Sounds silly but they seem to cause that problem. I just read where there are some kind of GM plastic covers for each of them to shield each from one another or other metal objects. I'm looking for an engine picture with them so I can see what these folks were talking about. When I first got my 93 the wires next to the dipstick tube was shorting. Wrapped them with electrical tape and it isolated the miss, I had the plugs and wires replaced.
Is it just a bad idle or do you also pick up a roughness during 1000-1500 rpm's? That could change things well it would make your situation more similar to what many of us are experiencing.
O2 sensors can be monitoring for voltage differences to see if they are working properly. Normally when its cool (I think it has to get to 600° before O2's kick in) then its a preconditioned set of parameters to get a cold engine started and running until the O2s feed the computer data to manage the air/fuel. But in that case if the O2 was misbehaving it would seem logical that when their data was being accepted then you'd see the problem in other words when it reached temperature. I'm not sure if 3-5 minutes is plenty of time for the O2's to get operational.
Could be fouled plugs. The AC one's are suppose to 100K, changed mine at 77K with the wires.
These are just a few quick checks that require only a small about of test and expense. Otherwise it can get to be more of a headache.
Sure are a few of us here that would like to solve this problem as well. Good luck!
Lani
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