: Randomly Creeping High Idle Problem

10-03-07, 06:10 PM
I am a proud owner of a 1998 STS and I am totally stumped, hoping someone out there can give me insight to why my idle starts to randomly creep up while driving. I can be at a stop light and the idle will start to increase slowly from 700+-rpm to 1200+rpm(in drive,2500+in park) trying to drag me thru the light. I also notice the idle creeps on the highway, I can take my foot off of the accelerator as if I activated cruise control. This seems to happen in warmer weather with A/C on and less frequently in the cooler days. Some days it does not do it at all. I have tried everything I could think of. I disconnected the cruise control cable, replaced IAC, TPS, throttle body, low pressure A/C switch, had Freon charged to exact specifications. I have been leaning in the direction that the A/C has something to do with it, but do not know what else to look for. Also, I have noticed that I can reset the idle buy turning off the engine and turning the key to the forward position (just before engaging the starter) 3 times. I have access to a tech2 if you could lead me in the right direction and I would welcome any ideas.


10-03-07, 07:16 PM
Have you (ever) cleaned the TB?

10-03-07, 07:37 PM
Yes the throttle body has been cleaned, even replaced with a known good one from another vehicle. The computer was also reflashed with the latest programming available.

10-03-07, 08:12 PM
Vacuum leak?

10-03-07, 09:48 PM
I thought the same thing and we tested for that...again this doesn't happen all the time, I would think a vacuum leak would be constantly high idle?? This happens completely at random times, more prevalent in the hot weather. I've been chasing this for over a year now and am completely baffled as to what to do. It will even stop doing itself if you drive long enough say like on the highway, like I said above when its doing it on the highway it feels like cruise control when you take your foot off the accelerator except cruise is not on (I disconnected that to rule it out). Then several miles down the road it acts fine.:hmm:
If you're at a light, shutting the engine off and cycling the key "resets" it and its fine, it could run the rest of the day fine or 5 minutes later start doing it again.
I'm stumped and several mechanics I've asked about this are stumped too, this place is my last hope....

10-03-07, 09:56 PM
:hmm: A real head scratcher. I'm stumped.

10-03-07, 11:20 PM
This is a simple problem.
With the Tech II look at TPS when the condition occures and look at the min or learned TPS. If it is not the same the problem is in the TPS circuit or throttle cable. If both learned and current TPS are the same and the Tech II show it to be at Idle (in the data list). Then look at the IAC counts. If they are High the PCM is driving the IAC open and increasing the RPM, But if they are low then you have a bad IAC. If the PCM data shows desired RPM 700 and the IAC counts are high The PCM may have an internal fault. :crying2:

I can go on and on, but this should be a starting point.

10-04-07, 04:23 AM
i've experienced a similar problem, sometimes my speed crashes to the point of stalling the engine.
i carried out the reset and learn procedures for the IAC and TPS, it does seem to sort it for a short while.
trying to find someome with the correct TechII here is very difficult and as the problem is intermittent (doesn't go wrong for days on end) it makes testing so much harder.
i am going to obtain a new IAC when i next buy parts from the US.

10-04-07, 05:12 PM
IAC... my 94 was doing the exact same thing and it got worse and worse. I would have to idle relearn to get it back to normal and unplug it so I could drive. New servo and I was back to normal. Probably a different system a few years newer.

10-07-07, 09:09 PM
Thanks for the responses, I will try plugging in the Tech 2 on the next warmer day and we'll see what happens.

10-09-07, 06:35 PM
I had the same exact problem, but the idle UP has only happened ONCE, A/C was on. I have a bouncing idle under coasting conditions.

I am hoping cleaning my tb will fix it, but I doubt it.

Post what you find and I will do the same!