Vibration at idle with A/C on


#1

I have an 04 Pontiac Bonneville with 48,000 miles. It has automatic dual mode air conditioning. I get a vibration when the car comes to a stop and the engine idles. It does not always do this. Usually when outside temps are 70 or below it’s fine. At higher temps it may vibrate as soon as the car comes to a stop and idles. Other times it will sit there for 5 minutes before the vibration starts. Any suggestions?


#2

The cooler the temperature, the lower the pressures in the A/C and the easier it is for the A/C compressor to turn. A minor shudder at idle when it’s very hot and the A/C is working hard isn’t abnormal, but the first thing to consider is when the last time this engine was tuned up. If the spark plugs, wires, air filter and fuel filter haven’t been changed since the car was bought new, now’s a good time.

An unusual or powerful vibration can be due to an improperly adjusting IAC Valve. That valve is supposed to crack the throttle open a little further when you turn the A/C on to keep the idle stable. I’d guess the normal idle speed is around 700rpm, it should be 50-100rpm lower with the A/C on. Anymore than 150-200rpm lower than standard idle is an idle control problem.

-Matt


#3

Take the car to Advance Auto or Autozone to have the engine computer codes pulled for free which will turn up any idle control problems.

-Matt (again)