Alright, just bought a 2005 scion tc and I seem to have a problem with rough idling while the AC compressor is OFF. The AC doesn't seem to work, but when I turn it "on" (like I said no cold air blows) it idles fine. Pretty strange so idk, any suggestions?

 

I'm hearing that it might be the idle control valve, any thoughts?

 

No idle control valve on these cars.
They have drove by wire throttle bodys so the ecu opens the throttle to control idle.

Best thing to do is clean the maf and throttle body.
Then unplug the battery for 10 to 15mins.
Plus it back up and start it and just let it idle for 10 to 15mins.

This should clear the fuel trims the ecu has learn and start the relearn process.

 

if that doesn't work, look for a small vacuum leak. at idle it will be rough. when you turn on the compressor it automatically ups the idle so if there is a small leak it will idle smoother with increased rpm's. just my opinion. good luck.

 

Do you use a K&N air filter?