Idle control and RPM

[Craig 94 TA GT]

Besides the IAC itself, is the idle controlled by another sensor, or just TPS, desired idle and the PCM?

I'm having a weird issue of the car idling really high after a freeway drive. When I get off the freeway, the idle will be roughly 1300 (or more) RPM and I have to stand on the brakes to keep it from moving. After a while at 0 mph, it will eventually lower down to 1200, 1100, then back down to the 900 desired idle. When it idles high, if I were to throw it in neutral, the RPMs would sit around 1800+. Also, if I were to let off the brake at the 1300 RPM idle, the car would practically drive itself - go through first, shift to 2nd and keep going forward, without touching the pedal.

I have a new IAC and that didn't change anything. A scan shows TPS is at 0%, and my AS&M 58mm TB does not stick open. I'm wondering what else would cause this...vacuum leak, another sensor, the high stall on the TC mixed with the PCM programming, etc?

Thanks in advance

[Injuneer]

Idle speed is adjusted by the PCM based on coolant temp, whether the vehicle is moving or not (VSS not = 0), whether the throttle is open (voltage above 1.0V), whether its in gear or in neutral (A4 only). Also keep in mind the IAC counts are just what the PCM is telling the stepper motor to do, not an actual position. There is no feedback. Have you tried the "reset" procedure on Shoebox's site?

When you come to a stop, and its idling at 1300rpm, what are the IAC counts? How do they repsond to the excessive idle? How low do they drop? Do they continue to drop slowly over time, corresponding to the slow reduction in idle speed? The loose converter may make it necessary for the IAC to respond more quickly to pull the idle down, since there isn't as much load from the converter, but it shouldn't prevent it from responding correctly.

[Craig 94 TA GT]

Thanks Fred, I'll get a scanner on it ASAP to see the exact counts and other readings again. If I recall, when the idle was high, the IAC counts were high. They gradually decreased from 140 to 80, going from high idle, back to 900. So the counts are gradually decreasing, but I'll get a scan to get the exact counts. My coolant temp is set to 187/187 and this problem happens during closed loop (not sure if it happens during open loop as well - I just may have not noticed).

I tried a while back to reset it according to Shoebox's site, but that was with the old IAC motor. I'll try again.

Once in a while too, it would even have the sensation of the throttle sticking, but I'm almost positive the throttle isn't sticking. Meaning, lockup is set to 45 so at that speed, it's around 1500 RPMs. When this problem happens, I could be at this speed and not even press the pedal. It would be holding this speed on its own. The CC was off for sure, and I'm certain it's the same idle problem because when I'd come to a stop, the idle was really high again.

[Craig 94 TA GT]

I can't seem to make this happen when I'm scanning it. Kinda frustrating. I also noticed that it could potentially do the same thing after running for a while, then shutting the engine down, and restarting after 20 minutes. I really don't know if it's just random now, or only occurs during these situations. I do have one scan where it did this very mildly, but only stayed around 1100 RPM, then decreased after several seconds.

[Craig 94 TA GT]

I just recently started having problems with the idle stumbling bad once in a while. I also noticed it stumbling under some load when locked up in 4th. At first I thought it was a plug wire. I replaced a questionable one but the problem happened again. It doesn't happen very often though.

I put the scanner on it and I think it may be the same problem as this thread started. The IAC count at closed loop idle, when stopped, is 0. I came to a stop and literally saw the counts go from 30 to 0. 70 mph is around 70 counts. Cold open loop idle is around 40. During 0 counts, TPS volts is 0.63, idle is 800 RPM and pretty smooth (when it doesn't do that stumble). MAF flow is in the low to mid 10s.

I already tried resetting the IAC using Shoebox's method, but no difference. I will double check the IAC screw, but I don't know if a MAF reading of 10, along with IAC of 0 would mean the blades are open, compared to a vacuum leak? Or is it somehow possible to have some kind of leak of after-MAF read air through the PCV and crank case?

[cold_surfer]

we are debugging a similar situation on my son's 93 firebird v6 with 144K.

I noticed the throttle body slotted-shaft going into the TPS was loose and worn. the aftermarket (grey part) was not ACDelco and it was getting varying voltages at idle and WOT -- but somewhat smooth with slow-roll-on acceleration. now we have new ACdelco IAC, TPS

We'reconsidering going with a new throttle body to eliminate vacuum leaks, shaft alignment problem, start the clock over for IAC carbon, and so forth -- it's $140 on the GM site they list above ($100 cheaper than my HOG discounted local parts store that is usually killer on prices). Thoughts on that topic? likely a waste of money ?? Ya'll please respond with your comments, thoughts, and experience.

once we get the idle and surging problem fixed we move on to the clacking valve-train noise. looks like prior owner did the timing chain already... I'm start at the rocker arms since that is what it sounds like most.