Sent: Wednesday, August 01, 2012 9:11 PM
Subject: Re: Hot idle speed oscillating
.... If you can't find it in the codes, it can only be a few things.
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.
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.
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.
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.
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.
So if you fuck 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 ?
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.