I posted this problem a few months ago and tryed many of the suggestions and have not come up with a fix.So here it is again. The idle will surge up to 500 or more RPM's when the A/C compressor cycles. I have cleaned the idle control valve then I replaced the idle control valve and that did not work so I put the old valve back on. I have checked for vacuum leaks and all is good. I think it has something to do with the signal from the a/c cycle switch giving an erroneous signal to the ECM making the ECM think that there is a bigger load on the engine. Does that sound logical? I have no parameters to check the A/C cycle switch to see if indeed it is bad. Any Ideas? Is the cycle sw serviced without discharging the A/C system?

 

What is your normal idle? My 1999 V10 is 650-700RPM when warm. 500 is less then a coil pack misfiring.

 

It idles about 650 in gear with the a/c off. I have had it jump up to 1200 when the a/c compressor comes on