Idle Fluctation while warm, last 30 seconds

[Earendil]

Symptoms:

• Idle fluctuates between 500 and 1100 rpm.
• Only fluctuates when warm.
• Idle suddenly becomes solid after 10-30 seconds. The "solid" warm state is the same 700rpm that it nails when the engine is cold.
• When in this fluctuation state, it will hesitate to come off idle.

Progression:

• This used to only happen at stoplights, and only one in 5-10 of them.
• The most noticeable change was during Friday's track day, where the idle would not settle down over the course of 60 seconds, and varied a lot more, maybe 400-1200 rpm and seemingly threatened to die on me.
• I took it back out again today and the original symptoms popped up, fading again after 15 seconds or so.

Clues:

• On the way too the track via freeway, I got a 1221 (O2 sensor) code that wouldn't go away (usually clears in a few seconds). While at the track the code remained (not unusual), and then vanished on the way home. I have not received the code since, nor was it common before the day I went to the track.
• Suspecting the intermittent idle problem was vacuum related, I had a smoke test done. Car turned up a clean bill of health with no leaks, though I was not present to verify the procedure.
• The TPS sensor checks out.
• The CTS shows 2950 ohm at 61f, and 255 ohms at 195f. This appears within spec.
• O2 sensor is only a couple years old.
• ICV currently buzzes, and was cleaned and proven to work 6 months ago.
• Car runs good and smooth. While I could be missing 10hp I suppose, I'm confident I'm not missing 50.

Current Guesses:
Heat is causing a sensor to act up, but which one? The problem appears related to heat. Whether that is coolant temp heat or engine bay heat I do not know. The coolant did not run hot while at the track, but of course the engine bay was significantly warmer than normal.

I'm honestly not sure how to trouble shoot what remains. I might take the car to a different mechanic and have it smoke tested again, but that's bordering on just "throwing money" at the problem. Plus, nothing in my current knowledge can explain why the idle can be so erratic and then boom, it's fine after sitting there for a bit.

Does anyone have any ideas? I believe I've run through all the basic trouble shooting and am at a loss.

[jlevie]

The hesitation off idle and varying idle speed are most commonly the result of intake leaks. A smoke test will find them.

[Earendil]

Symptoms:
[*]Suspecting the intermittent idle problem was vacuum related, I had a vacuum test done. Car turned up a clean bill of health though I was not present to verify the procedure.

[...]
I might take the car to a different mechanic and have it smoke tested again, but that's bordering on just "throwing money" at the problem.

The hesitation off idle and varying idle speed are most commonly the result of intake leaks. A smoke test will find them.

I see now that I could have been more clear. The way in which the "vacuum related" problems was tested was by doing a smoke test. There were no leaks found. I'll clarify my wording for future readers.

While it could still be an intake leak and the original test didn't find it, I'm trying to explore less common issues before paying to have a second smoke test done.

[jlevie]

Was the smoke test properly executed? That means plugging the exhaust, replacing the AFM with a plug & nipple for oil smoke injection, and holding a smoke pressure of 2-4psi for at least 5 minutes. I've seen techs (that I thought would have known better) not properly do the test and thus not find all of the leaks. Note that if any leaks are found, they must be repaired and the test repeated until no leaks can be found.