High idle when AC/heater is running
 

I have an 89 GTA. It had a built up LS1 motor from a 2002 camaro SS.

My issue: when I turn the AC/heater in. The RPM's spike up to 2500 RPM's. After a while and only if I let the car sit. The RPM's will drop to normal op range. If I am driving and turn ac/heater on enroute. Samething RPM's go up and stay at 2500 RPM's. Which is sort of nice when on the freeway. I have an M6 so it acts like a Cruze control.

Anyways I have already replace TPS. So I am assuming this continuing issue is somewhere else. Has anyone experience this issue before? Were you able to resolve a fix?

I expect you have a vacuum leak somewhere, which is
not enough that the IAC bottoms out trying to hold idle
RPM (or maybe it's still bumped up, but you just don't
gripe about a 900RPM idle, don't say). But when you
turn on the A/C, an idle airflow adder comes in for the
drag torque, and now you've got even more air than you
need.

I'd look for air leaks in things like the PCV boot, gaskets,
brake booster, etc.