Sorry Caddy Dudes!

[99BlackSTS]

Sorry guys but before anybody says "do a search" I just want to say I HAVE!
I spent all night last night researching and reading posts.
but it was hard to get answers i was looking for. we should make this a sticky.

Here it goes:

Problem: Engine revs when it is idle after short to long trip and will NOT return to normal rpms (600-750rpm)

Car: 1999 STS (4.6L)

Solution: Ive heard everything from throttle body to TPS to ISC.

What I have done thus far (and still no fix):
I have cleaned the throttle body (butterfly, bore, MAF sensor).
I replaced the Air Idle Control Valve (it was bad).
Now today (cause i found out yesterday your supposed to do this after a throttle body cleaning) I am going to disconnect the battery for 30 seconds and reconnect.
BTW - what can i expect afterwards when i do this.

so whats next?
a manual adjustment of the ISC abd TPS?

possible replacement of the ISC? ive also read that ppl have replaced this numerous times with no change.

"relearn" ISC? although ive read that this is pointless if you disconnect the battery.

I want to eliminate this problem, its getting very annoying and i can only imagine what its doing to my brakes and rotors.

Much love and respect to the Caddy Dudes who help me out!

[EcSTSatic]

I can only speak from experience with another car/make. If it is the TPS, I imagine it will need an adjustment or "relearn" process. They need some reference point to go by. In other words, you can't know how far you've traveled unless you know where you started. I had to use a meter to"zero" mine out.

Doesn't the manual have some R&R instructions? Sorry if I'm not much help.

[mcowden]

You definitely should disconnect the battery before anything else. I give it a minute or so then reconnect. Afterward, you should see a P0603 code in HISTORY indicating the short term memory was reset. That's good. Just drive it normally after that and it will figure out the correct value. If it doesn't idle right after a couple of runs, that didn't fix it.

You didn't say that you've checked for, found, or fixed any vacuum leaks. Have you listened for hissing all around the intake and/or used a water mist? Also, have you checked for anything that might be binding, pinching, or otherwise holding the throttle cable? PCV valve and hose intact? Is the MAF free of cracks and tightened against the TB with gasket intact? Is the brake booster line connected and in good condition? These are just a few simple things to check. There is more that I'll post later if none of this is a problem.

What occurred when this problem started? Did it just appear one day? How did it change when you replaced the IAC? Did it improve the problem at all? Were there ever any codes? Has anyone hooked up a computer to it to read the IAC, MAF, O2, and TPS sensor values while it's running?

[99BlackSTS]

Hey Michael and EsSTSatic,

Thanks for both your replies.

Michael,

I'm first going to disconnect the battery and I will get back to you hopefully with some good news.
But just to let you know, I am going to do all those checks you suggested.
In the meantime what are other things I should check if those checks are all OK?

Nothing happend to start this problem, just took it on a long trip (1.45 hours)
It appeared the second day i got it (as mentioned above)
When we cleaned the Throttle body (first thing i did) It was fine for a week. It usually happens almost every day near the end of the trip when i drive to work (45min drive).

After 1 week with no problems it showed up again.

We then replaced the Air Idle Control valve saturday aftenoon, and saturday late night it happend again when i got home from downtown (20-25min drive) and has been happening sporaticaly since.
We never hooked up a computer to it to read the sensor values.
But I will check the codes and values.
What are the correct values for those sensors you mentioned?

I will get back to you after i perform the first step (disconnecting the battery).
Thanks for your knowledge.
BTW- "WE" means me and my friend, he is a mechanic.

[99BlackSTS]

Well... after disconnecting the battery and all the stuff i did before (clean throttle body and replace Air Idle Control valve)

my Idle problem still happens. not as frequent as before. in fact i would say its a great improvement, but nonetheless it still happens. almost always after a long drive.

what else can i do, if all those things i have done have improved the issue?
where does the problem lie?

[lry99eldo]

So, what you are saying is that it improved (or went back to normal) after battery disconnect, yet returns after a long drive, right? Hmmm, some how sounds as though the PCM or related system is somehow thinking you are still on that long drive or has decided to calibrate or set another value. In any sense, it is a "system diagnostic" problem that you may not have too much control over. There must be a way of resetting other than battery disconnect if that hasn't eliminated the symptom/trouble. Because that's what it sounds like. I think you may want to plug that baby into a full system diagnostic to determine the parameter settings and hopefully it will point out the over idle anomole. I think that when pluged in the diagnostic it first reads what and where things should be as programed in and then runs a series of tests to on those actual component systems and then compares the results/actions. If something doesn't match up with the base information then through codes it tells you those systems that need replacement , adjustment, or further investigation.
lry99eldo