High idle after driving

[Nudnik]

Calling all Miata experts!!!!

Car: 1990 stock under the hood except for:

• Racing beat exhaust header
• high flow cat

My Miata is my fun car that I use to autocross & very rarely drive to work. My car operated perfectly for the first 5 or 6 times I drove it (total of ~300 miles which included a few autox events) & the problem has only developed over the last 6 or 7 times that I've driven it. When the car is cold it starts up as normal. It idles at about 1k rpm until it warms up (~ 2-3 minutes) & then it drops down to about 850 rpm (as it should). If I let it sit there & idle for over an hour (I did an engine oil flush) the idle stays at 850 rpm (even if I rev it up above 2k rpm a few times).

The symptoms happen if I take it for a drive (whether a spirited drive or a long 90% highway drive to work). It will usually hold a steady idle somewhere between 1700 & 2100 rpm (used to be steady @ 2k rpm but now it's idling at about 1800 rpm). It's not an erratic idle at all. It holds very steady (maybe bouncing less than 100 rpm) when it is idling high. If I turn the car off & let it cool down for 15 to 20 minutes, it will go back to idling normally until I take it for a drive & then the idle goes back to the ~2k rpm range. The only other symptom that might not be related is when it's idling & I stomp the throttle (while in neutral) the engine stutters/bogs (like it's starving for fuel) until it reaches about 2000 rpm & then it revs like normal.

I tried the non-invasive cleaning by spraying the air supply tube to the ISCV w/throttle body cleaner while the engine was running (no bueno) & I've replaced the coolant temp sensor behind the head (still no bueno). I replaced the spark plugs & spark plug wires (NGKs) in hopes of fixing the acceleration bogging (again, no bueno). The only other thing I can think is that I need to remove the TB & try cleaning the ISCV but thought I would ask here for any other suggestions before I went that far.

[OZMDD]

I would rule out simple things like vacuum leaks, hoses that shift under load, cracks in the crossover tube, and the status of the AC "kickdown" switching system. Also inspect the AFM carefully to see that its opening and closing fully with the airflow, maybe even try a known-good afm swappped-in. After that, you're going to want to talk to a specialist like Gary at TDR or Rogue if he is available.

[sammm]

Check the seals where the fuel injectors sit in the intake manifold. I found a vacuum leak there on mine.

[Nudnik]

Any recommendations or guidance on checking for vacuum leaks?

[sammm]

^Squirt some throttle body cleaner around intake side of the engine bits while it's running. You'll hear the idle change.

PS - Read this if you haven't already.
http://www.miata.net/garage/isc.html

[Nudnik]

^Squirt some throttle body cleaner around intake side of the engine bits while it's running. You'll hear the idle change.

PS - Read this if you haven't already.
http://www.miata.net/garage/isc.html

Are there any other places I should check for vacuum leaks besides the intake & spark plugs?

PS - not only have I read it (multiple times) but I've also printed it out so I can use it as my guide if I decide I to pull my TB to clean everything.

[sammm]

Did you test the ISC valve, or whatever that thing on the bottom of the throttle body is called?

[POS Racing]

http://www.dfwmiata.com/threads/26508-Random-idle-surge

I'm still thinking IAC.

[Nudnik]

http://www.dfwmiata.com/threads/26508-Random-idle-surge

I'm still thinking IAC.

That is my next step but I thought I'd ask one more time before I go that route just in case somebody else had any other ideas. It's good to know your consistent in your diagnosis though.

[POS Racing]

Maybe if you ask the same question a third time I'll come up with something else - but I doubt it.

[OZMDD]

Ooh, ooh! I wanna try! Have you checked the IAC??

[POS Racing]

Ooh, ooh! I wanna try! Have you checked the IAC??

LOL!!

[OZMDD]

Are you sure it wasn't YOU who was high while idling?

[Nudnik]

Are you sure it wasn't YOU who was high while idling?

LOL!

Mike are you advocating DUI?

[OZMDD]

I said idling. Alone. In Colorado.