I think I basically understand what the LH (through the ISV) is doing at idle on a warm engine (opening and closing rapidly to maintain the target idle speed). I think it also comes into play on decel...it closes until rpms drop and then resumes control? I'm iffy on that one.

What I'm most curious about is what it does at cold idle. I think I read where the LH doesn't control idle for a couple minutes after a cold start. Since RPMs are generally a little higher until the engine warms up (I think? My car hasn't been right in this area for a while) I'm guessing that means the ISV is left open, same as when you jumper the two diagnostic connector pins. Is there ever a time when it closes it and leaves it closed? I think there is a vent in the throttle in case the ISV is closed off through some fault, to it can idle, but is there a time when it closes it off intentionally? Maybe that's decel until the rpms drop?

Or do I have this all wrong?

Btw this is for an '86.5 S3, if different years behave differently in this matter....

 

My understanding is once the engine reaches some temp (warm but not full operating temp) it doesn't affect fuel/air flow.

 

Hmmm, I thought it was the opposite...