I just installed a new Gm engine in y car and ever since, the car idles at 2000RPMs and jumps between 2000 and 1700 all the time at idle. it sounds like i have a lumpy cam in it. I have installed a new IAC motor and tried messing with the idle screw but nothing helps. sometimes maybe 1 in 50 times i stop it will go down to about 750rpms. i have even backed out the idle screw all the way and the car still idles that high with the butterlfies closed all the way. also. when it is idleing high like that I will release the clutch slowly and the engine bogs back down to about 600 but as soon as i push the clutch back down the engine revs right backup again. what gives.sorry its soo long.The IAC motor is new from the dealer. engine is a 350 tpi speed density. mods are in signiture.

 

if it will idle that high with the butterflies all the way closed it sounds like a major vacuum leak.

 

I checked for vaccum leaks and cant find any. I cant hear any hissing noise coming from under the hood either. and sometimes it idles fine. i cant see hoe it idles fine on time and otheras it dont. the throttle isnt getting stuck either. i have a throttle return spring on the TB

 

I have a very similar problem. Mine was idling too low when I Drive so I turned the idle up and adjusted the TPS. Now when I go for a ride and come back and put it in park it will run at 2000 RPM's and only come down after turning the car off.
Once I restart it it will idle in Park at like 1200. I'm gonna put a vac gauge on it, but it only does it when the idle is turned up to 1200, at 1000 and lower it doesn't do it.