Hey there folks, I have an 89 Trans Am with a high idle problem I could use a hand with. Here's the quick rundown:

History: Barn find, bringing back to life. Just replaced fuel injectors and fuel pressure regulator due to faulty OEM injectors.

Symptoms: When the car first starts it idles at ~1100RPM, idles cleanly but bogs down with surging and backfiring under moderate application of throttle for first ~5 minutes of operation. After those 5 minutes, car runs well and has plenty of power, however idle slowly rises over the course of an hour of driving to the point where it sits at 2000-2800RPM. At times I have been able to physically pull back on the gas pedal to lower the idle to 1100-1500, but this ceases to have any effect after about half an hour of driving.

Troubleshooting steps: Searching this forum for high idle issues it seems the regular culprits are the O2 sensor, IAC valve, TPS, vacuum leaks and the mechanical idle adjustment screw. Here's how I approached each of these.

O2 sensor: replaced. It was cheap and easy to do so I jsut did it. Old one did not appear fouled.

IAC valve: Based on my limited knowledge, this seems like the most likely culprit. Odds are decent it was damaged when we cleaned the throttle body while it was removed to replace the fuel injectors and when i inspected the valve it was fouled up pretty badly and did not appear to actuate. I have replaced the IAC valve and followed both the factory service manual instructions for calibration and the technical information here about resetting the valve, but neither method made a difference with regard to the high idle issue. I have not yet but will this weekend pull the IAC valve to verify it is functioning, I did not verify function of the new unit before installing it.

TPS: Set to 0.54v at 0% throttle as per the instructions here and the factory service manual

Vacuum leaks: Steady and strong vacuum under normal operation

Mechanical Idle Adjustment screw: Has reached the extent of it's travel with no effect. Reset just to the point where there is no gap between the screw and throttle plate lever.

Other: I was concerned we had flushed out the lubricant for the throttle plate lever when cleaning the throttle body, so I relubricated that, it moves very smoothly and has a strong return when the engine is off. When the engine is on however there seems to be too much resistance for a smooth return by the spring. Even so, manually closing the throttle does not drop us below 1100RPM.

 

Posting the solution in case Google brings anyone here for this issue.

The issue wound up being the result of the combination of 2 factors. It turns out I had a bad coolant sensor, the result of which is that when the engine was cold my timing was advanced by 40 degrees, causing the car to run very rich and poorly. The other issue with this was that having just replaced the fuel injectors and reset the ECM, the car needed to relearn some things to bring the high idle back under control, BUT because the coolant temp sensor was dead, the car never went into closed loop so this process never happened.

I hope this helps someone in the future. A major help in this was the TunerPro RT software, I highly recommend it. I had spent a lot of money on parts and time on diagnosis prior to getting an ALDL cable and the software, but after getting those it was a 5 minute process to diagnose my issue.

 

Had a very similar issue. For a cold start, the initial idle was way too low... like 500 rpm and the car would barely stay started... then, fairly quickly the idle would float up to 1,000 rpms. Once up to temp, after a drive, it would continue to idle higher than normal, lazily wondering between 1,100 and 1,200 rpms. If I restarted the engine in this condition, while at temp, the car would start around 900 rpms and after 30 seconds or so, float back up to 1,200 again.

In my case, I found that my IAC valve (that had been previously replaced) was not fully tight.
I put a fresh IAC in, tightened it good, and it solved my problem!
Be sure to follow the instructions with the IAC. Measure for proper distance between the housing and the tip of the pintle. Also follow the instructions and the correct sequence for resetting the IAC.

Hope this helps someone out there! I can see from the many comments here, there could be multiple reasons that cause a similar idle issue. But again, in my case, a properly installed IAC fixed my situation.