It could definitely be either a dirty IAC valve or a defective IAC valve.When you turn the ac on,the IAC valve is supposed to increase idle to compensate for the load that the compressor puts on the engine.I would remove it and clean it thoroughly with carb cleaner,dry it thoroughly, reinstall then do a base idle reset.The link below will help.If you've already got the idle speed set correctly,you can let the engine idle then disconnect the wiring harness from the IAC valve and the engine should stall.If it doesn't stall,this can be a sign the IAC valve is defective.If you disconnect the wiring harness and connect a ohmmeter across the IAC valve terminals, it should read 7-13ohms.If you reverse the ohmmeter leads on the terminals, it should read infinite ohms because its got a diode inside of it.Leave the harness disconnected,touch the (+) voltmeter lead to one of the IAC valve terminals and touch the (-) voltmeter probe directly to the IAC valve case.Resistance (ohms)should be high.Measure both terminals the same way.If the meter reads 0-13 ohms,the IAC valve is shorted to the case,so replace it.The directions shown below are for a base idle reset.
Disconnect battery & turn on headlights for 5 minutes to erase KAM memory.
With the engine off, install a .025” feeler gauge between the idle stop-screw and throttle lever and adjust for that .025” gap.
Run the engine for about 30 seconds at 2500 rpm.
Return to idle and set the idle stop-screw for 650-750 rpm.You wanna keep the rpm level at or below 750rpm to prevent idle surging & rolling idle.
Turn the car off, restart and let idle for 2 minutes.
If idle is within specification you’re done except that you should pull the IAB connector to see if the engine dies. It should.
91 LX 342ci(10.5:1)
TFS 190cc(stage ll)
TFS track heat
XE276HR .544" .544"
Flowtech 1 3/4"
Prochamber w FM