My AP1 started to throw the IACV code (I think it was p0505) and having a VERY fast bouncing idle. The idle starts up steady about 1500 and steps down to about 1200 after a few seconds. If I rev, the idle starts to violently bounce between 1400-1600. Like three or four bounces a second. The entire car shakes a little because of the revving.

If it's a warm day out (85F+), I can get it up to temperature, turn off the car, start it back up, and the problem is gone (no bouncing, sub-1000rpm idle). If it's a cooler day out, sometimes after I stop and start the car at temperature, it still will bounce.

I cleaned the IACV with carb cleaner (yes, I got the 5-point Torx screw and took off the motor part). The valve(flap?) inside seems to spin without impediment.

Another strange symptom is that when the idle issue is happening, the low speed throttle control is SUPER jerky. For example when I am below 3-4k rpm in 1st or 2nd gear and I let off the gas just a little, it jerks very hard. This symptom goes away when the idle issue goes away on hot days.

I'm guessing I have to just replace the IACV, but I wanted to check with the S2KI experts before I do.

Thanks for reading!

 

It could have bad gas in the tank. Run a bottle of injector cleaner first to rule that out before replacing the IACV. Apart from that, you might have a grounding issue. Check the condition of your battery's negative terminal and black ground wire running down to the subframe to be sure its not in bad shape.

 

Thanks, I will check those two things asap.

 

I doubt you'll need to replace your IACV and out of the 10+ years on s2ki, "bad gas" has never actually panned out in anyone's troubleshooting.

My problem is with your quote -- "if I rev."

Did you reset your ECU afterwards and then start the motor, and let the car finish its relearn procedures (e.g. 5-7 minutes of idle without touching the gas pedal at all)? Stop touching the gas pedal trying to stabilize the idle or save it. Reset it and let it learn on its own.

Concurrently, other easy items to check -- is your intake filter newish/clean? Throttle body cable is properly tightened?

 

After I cleaned the IACV and disconnected the battery to reset the ICU, I followed the procedures for having the car relearn it's idle. I thought it was fixed after that, because no light came on after I let it idle for about 10 minutes, and I didn't have any issues during the subsequent drive. The next time I started it up, though, the CEL came back, and as soon as I hit the gas, the idle issue was back.

My local S2000 expert did clean my throttle body and tighten my throttle cable recently, so maybe it's throttle cable related.

Intake is a Comptech which has not been cleaned ever, as far as I know. At least not in the 10,000 miles I have driven the car. I'm looking into threads about that now.

Thank you for the guidance!

 

What's frustrating is that almost all the threads on s2ki about p0505 are left unanswered or abandoned by the original posters. Ugh, people.
https://www.google.com/search?q=p0505+site:s2ki.com

One pattern everyone mentions is to also check for vacuum leaks, so add that to your list next. Makes sense.

 

I went through as many of those old threads as I could to find a silver bullet solution before posting, but no such luck. I showed this thread to my mechanic/S2K expert, and he said this could be a symptom of the IACV dying or simply low coolant levels, which I haven't checked yet.

 

I had a case of bad gas in the tank or rather, the tank getting dirty over the years of previous owners never doing any maintenance in the form of fuel system cleaning so when I ran a bottle of injection cleaner when my car had a misfire, it made a significant improvement to it and improved idle quality. That wasn't the only reason why it misfired (coils, tube seals, etc) but the cleaner still made a difference.

 

I replaced the IAC unit and the problem has been resolved.

Here is the part number if anyone needs it: 36460-PCX-003

 

So much thanks --- for posting the cure! I hope you never have idle issues, again.