Nhogan84

I have an issue with my 2000 mustang GT in the way that when I sit idling when the A/C is on, it fluctuates in its idle. When the A/C is on its lowest setting, it'll fluctuate from about 400-500 up to 800-900 and back. When its on a higher setting, it will fluctuate from 100-400 up to 800-900, sometimes fluctuating so low that it will stall. Under the hood, I watch the A/C compressor clutch will sping for a few second (when it fluctuates high) then stop spinning (when it fluctuates low). It idles just fine at 500 when the A/C is off. I have no idea whats going on here, any ideas?

cliffyk

The IAC (Idle Air Control) valve is sticking and needs to be replaced...

Nhogan84

Doubtful due to it idling just fine when the A/C is off. I'm really thinking that the fact the A/C compressor clutch only spins for a few seconds then stops for a few is a big deal. I'm not sure, but it just doesn't seem right. Especially since when its spinning, the idle jumps but when its not spinning, it plummets.

cliffyk

The IAC is, as its name implies, what controls idle speed--it and nothing else.

The only caveat to that is that the initial setting of the throttle stop screw (it is NOT an idle adjustment screw) needs to be properly set or the IAC cannot do its job properly. The stop screw and its seat can both wear as the years go by and allow the butterfly to close too much so that the bypass air is insufficient, or if someone may have mucked about with it thinking it was an idle speed adjustment.

Here is the procedure for resetting the stop screw.

Also, the instrument cluster tach is not dead on accurate at idle speeds, however 500 rpm (in drive) is a bit low for a stock tune--typically for the autos the idle in neutral is 700 +/- 50 rpm, and 560 to 600 in drive.

keeno

Usually when the ac compressor clicks on and off every few seconds (causing the fluctuating idle speed) the refrigerant level is low and needs to be topped off...no more engaging and disengaging every few seconds.

Patient957

I just replaced my compressor and this was certainly the case. It would turn on for a split second and then disengage. Gradually as more refrigerant was put in it would run longer and longer.