i have a 1990 atomatic miata that is in perfect condition. it has 78,000 miles on it and runs like a dream. when i sart the car it idles at 1500 rpm and if i let it sit and warm up it sits at 1000rpm but after driving it for about 15 minets and putting it in park it idles at 2000rpm. i was wondering if any one else has this problem and what was done about it.

 

sounds to me like you need to reset your idle.
follow this guide here
http://www.miata.net/garage/isc.html

 

There's a way to test your idle valve, which I think is the same on automatics. Look on mt.net or m.net for the procedure.

 

I wouldn't touch any adjustments. The idle valve should not need adjusting unless major mechanical changes have been made to the engine. It is far more likely that your twenty two year old rubber vacuum lines have been baked until as brittle as pottery. I would start with an exhaustive search of every inch of each of them. A stuck PCV valve can allow a direct leak as well. It needs periodic replacement anyway, so if you haven't yet changed it you should.

Carburetor cleaner works well for finding leaks if you know the process.

And don't discount the chance that your IAC or EGR might be stuck or carbon encrusted. A removal and thorough cleaning may be in order.

 

worked on the car tonight and changed the pcv. and checked all vacume lines..... all looked good and pcv change made no difference but thanks for the info. so i moved on and decided to ajust the BISS (bace idle st screw) and droped the idle when hot to around 1200 rpm and that sounded about right so we'll try that for now thanks for the help.

 

idle is usually 900-1000