It's probably due to the A/C compressor engaging. You may have a bad IAC ( idle air control) but try cleaning the throttle body first. Remove the intake snorkel, have someone hold the throttle wide open for you and scrub the back side of the throttle plate and surrounding bore with an old tooth brush and some carb cleaner. Be sure to spray some into the small holes next to the throttle plate. That should help stabilize the idle. If it still has a problem, replace the IAC.
June, 15, 2011 AT 8:24 PM
I've been fighting the same problem for the last year plus. In time you will likely get a trouble code setting off the check engine/service engine soon light. The trouble code will be for the cam shaft position sensor and the crankshaft position sensor. I was Unfortunate enough to bring it to the wrong dealer for the first 3 tries to fix the problem and got ripped off. With research on line and talking with several mechanics I was told and found out one sensor will set off the other and both will need to be replaced. They are not too expensive, 80 U.S. Dollars for the two parts. After the sensors are changed the PCM will need to "relearn" the timing of the engine. This almost has to be done at a GM dealer, and cost about 80 U.S. Dollars. Total time to replace both sensors should be 1/2 hour. The cam shaft sensor is very easy to change, the crank shaft sensor is a bear to get at. I choose to change the cam sensor myself, but couldn't reach the crank sensor. Change both sensors and have GOOD DEALER relearn the computer system. Mine seems to be working for now, best of luck.