mfpthebronze

I spent last Saturday trying to track down a high idle issue, and thought I would post what I found in hopes it might help others in the future.

The car is a '99 with AC and PS, DIYPnP MS2, and a cobbled together turbo setup that I'm in the process of finishing up.
I started the car on the preceding Friday and got the idle dialed in at about 800rpm, everything humming away nicely. Started the car the next morning and it was Idling at 1400rpm.

1. I re-flashed the previous day's tune to make sure something hadn't gone wonky. Nothing.
2. Verified fueling and ignition timing, even checked for pre-wideband exhaust leaks that might be throwing off my fueling readings.
3. Checked for vacuum leaks visually and by spraying carb cleaner liberally around engine bay and around injectors listening for idle changes. Nothing. (WARNING: Spraying carb cleaning everywhere can make things go boom)
4. Removed BOV and sealed port to make sure there was no air getting in and messing things up. Nada.
5. I tested the IAC valve in "Output test mode" in Tunerstudio. Would raise the idle even higher, but not drop down below 1400rpm.
6. Fully closed air bypass screw on throttle body and fully backed out throttle body stops. Idle dropped to 1250rpm.

At this point I knew air HAD to be getting in after the throttle body somehow, otherwise step 6 should have killed the engine. I pulled off the throttle body and upper half of the intake, cursing myself for reusing the old upper intake manifold gasket like a cheap idiot.
Reassembled the manifold with the new gasket I had in waiting but had opted to not use last time. I know. I know. Started the car up, aaaaaaaaand.... still high idle.

Busted up bench and knuckles later, I pulled off the throttle body and IAC again. Pulled the IAC to clean it of any possible deposits and found this:



Looks like an aluminum shaving from when I had tapped the intercooler for IAT sensor had somehow escaped my cleaning efforts, and made its way into the IAC. getting stuck between valve and seat
Removed it, mounted everything back up, and still had high idle.
I had to test the valve again in tunerstudio and slam it open and closed 2 or three times to get the valve to finally seat in the correct position.

Idle finally came back down, I adjusted throttle stop and air bypass screw again, and it's ticking over nicely at 800rpm.
Moral of the story? Don't turbo your Miata.

And double check for FOD.

Davinci

Thanks for sharing, I am having the same issue.

Question 1: Is your iacv open to the atmosphere or is it shut with some kind of plug?
Question 2: Exactly how do you test the iacv in tunerstudio? I cant seem to find the setting.

Thanks in advance

Mudflap

Are you 100% certain you don't have a throttle cable lightly pulling on the throttle body? Remove the throttle cable completely from the body to make certain.

Davinci

I am not 100% certain of this, I actually feel really stupid now. Wow. I just went to bed so will have to check it tomorrow.

How is your IACV hooked up? I have heard a lot of different ways, some people say "slap a filter on it" some people hook it up to the inlet piping and some people remove it. I dont get it. If I let it suck air through there its gonna go very lean