When my car is full operating temp, when i am at a either complete stop or very slow speed, if my car is in neutral the rpm will steadily fluctuate from about 1500 and 500. if i put my car in any gear it'll become steady again. it was doing when i first got it, and since i've done a number of upgrades (i.e. turbo, radiator, afpr, injectors, k&n intake filter, fuel pump, emb, etc.). Any ideas on what it may be?

 

any change if you unplug the idle valve?

 

haven't checked that, didn't even know about it till you said something, but i found a pretty nice bit in the miata.net garage. so is there an easy way to check that before i spend more money on this stuff?

 

Sorry to resurect the thread, but incase you haven't figured it out or for anyone searching this happened to me too. In my case it was the clutch switch under the dash that gets pressed by the clutch pedal. If you look up under there there are two. In my case it was the one that was closer to the floor that is released when the clutch is pressed. It allows the ecu to regulate the idle when not under a load. I don't have a part number but the scematics at the mazda dealership made it easy to point out. Hope this helps anyone searching.