warm start idle problem

[vtlaxplaya16]

Alright i have a 99GSX this problem started a few weeks ago, i have no idea what it is, when i start the car cold, itll rev up to about 1500 than after warmed up itll fall back down to a constant 700, but say i take it for a 15 min drive go into the store and come back out and start it back up the rpms will go up to like 1000 and than fall down to like 400-500 and sometimes even stall out, when this happens i usually have to give it gas to keep it from stalling out.I have to start driving it before the rpms will stay constant. Any help would be appreciated.

 

[CurtTSI]

I had the same problem with my car. It was caused by a bad ISC. The ISC worked and tested good when cold. However, after being warmed up, the engine would die when off throttle. When coil resistances were tested in a warmed condition, one of the ISC coils showed extremely high resistance. Cool it down, and it tested good again. So, test the ISC coil resistances after warm up and with it still on the car. I ended up rebuilding my ISC from good parts from 2 bad ISC servos.