Had a somewhat strange problem recently that I thought the group might like for their collective knowledge base.
My Miata would start fine when cold, or had sat for a while, and would run fine for short trips. When it was fully warmed up, it would occasionally develop a erratic idle. It could be too high, or drop down and stall out. When this condition occurred, it would also run a little rough. The most noticeable problem would be that it would bog down on quick acceleration. If it was turned off when these symptoms were present, it would be very hard to start. I would act as if flooded, and the throttle would have to be held to the floor to get it to start. sometimes it would then take a while for the idle to smooth out again to be driveable.
Well it finally got to the point where it the condition was consistent after it was warmed up. The most noticeable oddity was that the idle would go to about 1500rpm with the shift in neutral, but would drop back down near normal if put in a gear. Until the engine was to full operating temp, it would be pretty normal acting, with maybe a couple of odd spots during acceleration.
So, definitely temperature related. But, there were no error codes reported by the ecu when trying to do diagnostics. Tried an alternate air flow meter with no real change. Then changed the engine temperature sensor. The one on the back of the head by the coil packs. Not the fan temp sensor, or the instrument temperature gauge sensor. That did the trick. Now runs like it should.
So it appears the engine temp sensor can have a failure mode that does not register with the diagnostics. It appears it would short out when the temperature was very near the normal operating temperature and would then act like it was very cold. (Could be an open condition also.) Result was it would run a rich mixture causing the other problems. Just a troubleshooting note to add to the collective knowledge. Will post to the maita.net forums when I get a chance.