Idle speed high then slowly comes down.
 

My 91 zr1 seems to hold a high idle (1200) when cold then drops down to the 700-800s where it should be. as it warms up it slowly goes away. Somewhere I read there is a prom fix for this. How to get it? How much? Or is this a vacuum leak and we should start thinking about a pull of the plenum? Your thoughts!

My idle stays at 1000rpm's until I come to a complete stop than it drops to around 700 it’s a little slower to drop with the AC on as far as when its cold that sounds about right.. If you have a vacuum leak, you will hear the pump running with just the key on engine off. It should come on when you first turn the key on than cycle off.. You may hear it cycle about every 60 seconds this is normal. If it runs continually, you have a vacuum leak..

so this is normal. wait ac is not on at the time. Gotta check an ac relay as the compressor is not kicking on. fuses are fine and no codes on the console for ac.

High idle when you come to a stop and then drops down is usually a PROM update issue on '90 and '91 ZR-1's. See http://www.zr1.net/ZR1_issues.html#2for more info. A new PROM is around $50.(Edited by ZR1Mom at 2:51 pm on Dec. 3, 2002)

I checked I have the updated chip...I have noticed after changing to the Flowmaster system the idle stays up at 1000K until I come to a full stop.

It also could be dirty throttle body plates or dirty IACvalve.

Thanks.. any tips on cleaning.. can't be to dirty only 25K on her..

Take some brake cleaning fluid. Remove the horn on thethrottle body and flip the plate up and look behind it.I'll bet it is pretty dirty.