My 2001 gt with a manual transmission once warmed up idles around 700-800 rpm which is great. When I turn on the A/C the idle stays at 1100-1200 rpm. Isn't this to high?
Does the AC compressor clutch short cycle when the AC is on?
The PCM maintains two IAC idle trim settings. One with the AC on and the other with the AC off. IF the AC is low on Freon causing the clutch to short cycle, THEN the PCM is unable to learn new AC on IAC idle trim settings.
The other "possible" cause is a charging system problem such that the battery isn't keeping the PCM "keep alive" circuit hot. This causes the PCM to "forget" and restart the learning process.
A possible work around is to temporally "jump" the low pressure cycling switch to force the AC to run. The idea is to allow it run long enough for the PCM to re-lean new AC on IAC idle trim settings. Do not force the AC to run too long as it could make the AC coil freeze.