I'm hoping someone on here can help me with a strange problem I'm having with my 93 mx5.
I've had the car for years and the idle speed has always been a steady 900 rpm when warm and about 1100 rpm when starting from cold. Recently when starting from cold the idle is about 1500 and then rises to 2000 as soon as I start off, it will stay at this level for about 20 - 30 mins of driving before dropping back to normal. Also when I turn the AC on the revs will rise even higher to about 2500, once the revs have finally returned to normal the AC then operates correctly with the idle speed just picking up slightly when it is turned on.
The temperature gauge operates normally during the warm up period with the car reaching normal operating temperature after about 5 mins.
I am thinking this must be a problem in the idle control system but don't know where to start, any help would be greatly appreciated, thanks.

 

Check for vacuum leaks between the MAF and the throttle body, then all of the vacuum lines to the intake manifold.

 

Blow out the water lines going to the Idle Air Control Valve (on top of the intake manifold) to make sure you have good water flow. If this doesn't change anything, take it off and clean it so that the piston can move freely with temp change.

Also, check to see that the rubber cap on the nipple at the extreme rear of the intake manifold is not split, cracked, loose, or missing.

 

Ahh, yes, the cursed water nipple.

 

No, the one on the manifold. It can blow off with a turbo and needs to be clamped; but can degrade on older cars to a point that it causes a vacuum leak.

You are thinking of the one that leaks water about 3 inches away.