Stupid Idle

A few months back (on the old board) my 1994 1.8 liter Miata was diagnosed with an idle problem. First, it started to have trouble idling and ran on the verge of stalling out. After a week or so, the problem went away. Another week saw the current problem come into play - a fast-idle situation. It started with 100 rpms above idle, but it would creep to a higher number each day until it finally settled at a 2,000 rpm idle. To remedy the situation, I simply unplugged the idle air controller.

While this actually worked, I still have a problem since I can no longer use my A/C while idling. Here's what I have noticed.

If I unplug the IAC, the car idles fine for the most part, but dies when the A/C is on and the accelerator is not pressed.

If I plug the IAC in ever so slightly, a loud mechanism comes on that makes a whirring sound much like a fan (but it's definitely not one of the radiator fans). It sounds like it's coming from behind the driver's side headlight fixture. However, the car seems to work just fine - you can even turn on the A/C and it does the normal-car routine of snapping on the IAC and idling a few ticks above idle to run the compressor. However, when I turned the A/C off, it must have jiggled the cord and the main problem of the fast-idle resurfaced. Also, I could not replicate this situation again.

An interesting note: when I have the IAC plugged in and I turn on the A/C, the RPMs jump even higher as if it was just opening as it should - maybe something else is the culprit?

I have unplugged the battery for an extended period of time for other work hoping at the same time that the computer might reset. This was to no avail.

I've consulted the almighty Hanes manual for the car, which, of course, sucks. The only recommendation is to adjust the idle screw on the throttle plate. Does anyone think this is the culprit? What if I did this?

Something that might help me understand this mechanism a little better: since this car has a two-wire IAC, is the IAC either ON or OFF, or can it be in any position between? Is it hard to get off the car if I need to clean it? I cannot buy another one - Mazda has them for way over $500.00 (which is retarded) and Advance Autoparts has them for $830.00 (I'm not joking, somebody was smoking crack when they thought of these prices)! I actually don't even think the IAC is the problem anymore.

Insight would be appreciated. Thanks!

It's been a while since I've had my miata so I don't remember the specifics but based on what you described I'm thinking it's either a bad IAC (could be stuck) or a bad coolant temp sensor. Oh and get the Rod's Miata Enthusiast Manual.

As for parts check ebay or junkyards.

What would a bad coolant temp sensor do for the car? Where is this sensor?

I just went down this path on my 318is. Suggestions I found were IAC, or on the BMW ICV, vacuum leak, or oxygen sensor. I tested the ICV's resistance and found it lacking. Replaced it and it idles again.

www.autopartswarehouse.com was awesome by the way, ordered one after noon and it was waiting for me the next day when I left work.

I think the two wire IAC's can be pulse width modulated. That might be the fan like noise that you here. Also most need cleaning every now and then. I'd try that before anything else.

I don't know anything about Miatas but this thread seems to have some good info.

http://forum.miata.net/vb/printthread.php?t=265594

I advise against messing with the idle stop screw. It's called the 'don't touch it' screw for a reason!

I lean toward a sticky IAC. I have seen them brought back to life with plenty of carb cleaner and a toothbrush.

Jensenman, now that you mention it, it wasn't until after I sprayed a bunch of throttle body / intake cleaner in there that the idle problem switched from a slow idle to a fast idle.

I'll pull it apart tonight when I get home.

Thanks guys. I'll keep you informed with what I find.