From sci.electronics.repair
EXCELLENT info. J understood the problem right off the bat and that's = exactly what it was. The guy's a genius!
Hold on, f*ck all this. If you can't find it in the codes, it can only = be a few things.=20
Dig up, the first check is to keep it running until it does it. If it = won't do it out of gear get some really good wheel chocks. When the idle = hits a high point, unplug the IAC (or it's equivalent). If the hunting = stops there are one of two things possible, but one solution might do = it.=20
More than likely the IA path is partly clogged, especially where the = pintle goes down. This skews the actual IACS from the true reading, = indicating to the computer the amount of IA is higher than it is.=20
The IAC responds based on presets in the computer so therefore it will = cut the IAC postiton to where it does not give enough IA. Then the = computer goes into "stumble" mode, thinking (rightly) that it is about = to stall because the idle speed has dropped too much. It will then open = up the IAC to allow more air. The computer will always look for the = lowest idle attainable. This sustains the oscillation. (hunting)
Now, if you rebuild the throttle body, it will be cleaned and hopefully = be alright but then there is a possibility that the seat for the pintle = valve has been eroded too much. You usually cannot machine this out = because you cannot usually readjust the IACS to compensate.=20
But if you see a bunch of black shit in there of course clean it = somehow. Throttle bodies really never need much of a rebuild other than = that, if it's worn you might have to machione down a bore so the IAC = drops lower into the hole. (same shit happened to carbs that were run a = long time without an air filter, but the solutions were different)
However, if it is actually worn like that there is a workaround. There = is a tang on the throttle linkage right there on GMs, it may be hard to = find. On an American car it is a tang and you just bend it, on other = cars you might have to shim it, or even find the actual adjustment. This = will keep the throttle from closing too much. You MAY have to do = something about the TPS if it is not adjustable.=20
The drawback to not replacing the throttle body for this problem is that = the IAC pintle might get thrown down into the seat so hard the it will = get stuck there, and then you will have a stalling ocndition and a = constant check engine light.=20
So if you f*ck with the idle position of the throttle, do it only as = much as you need. (both of these solutions are against "code" but legal = if it pases emissions, and it should)
If you don't do something though, it will eventually start stalling at = red lights. Alot of people told me "well it was doing that idle thing = for years and now it stalls". What, did you think it was going to fix = itself ?=20
But it is not sensors. It is probably not the IAC itself, it is that = things have changed for the IAC. The parameters of the intake cuircuit.
J