Im having a somewhat annoying problem with my 91 hatch, 91 LS swap obd0 / pr4. The cold high idle stays up most of the time, about the time its hot enough for the fan to come on the idle will drop to about 750. After low RPM driving in town, or a few minutes of idling the idle will slowly work its way back up to 1200. now, if i rev it to 5 or 6k rpm once, the idle will instalty come back to normal for a few minutes, then slowly go back up over time, giving me the idea that its moving hot water through the throttle body again and letting it idle back down then cooling off and raising the idle. Ive checked for restrictions in the water lines to the fast idle air valve, seems to be flowing just fine, but it acts like its not going through fast enough, since i can rev it one good time and the idle will come back down. Its not a major problem but gets annoying just because of the vibrations at 1200 and hard urathane mounts. anyone else had a similar problem? short of changing the throttle body, which i cant seem to find one locally id like to get this straightened out.

 

Try cleaning the idle control valve. They gum up and stick, and disrupt the idle setting. Give it a very light lubrication after the clean to allow it to move. An overnight soak is normally good for removing stuff, or throttle body cleaner.

The water should not affect it - it's there to make sure the low pressure through the throttle body and idle control body doesn't cause icing in low temperatures.