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Q: Car vibrates/shakes when sitting still in idle (in gear or in neutral).

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After having my car serviced for oil change, tire rotation and air filter replacement, as I was leaving the service center my car had a new engine vibration/shake while in park or neutral in idle. If I rev the motor to 1000 rpm the problem seems to go away. I am not getting any engine codes. I have since replaced all motor and transmission mounts (3 were broken anyway) but this did not make any difference. I have taken the car to Roger Beasley dealership who had it all day and could not discover the issue. It drives fine. Any electrical load while sitting still seems to make the vibration worse. Lights, fan, AC, power windows, windshield wipers, even pressing the brake pedal. This is driving me crazy not to mention making me concerned that something serious is about to happen. Any ideas?

My car has 94000 miles.
My car has an automatic transmission.

A: This sounds as if you have a low idle condi...

This sounds as if you have a low idle condition. I can say your car isn't likely to suffer a catastrophic failure because of this. With a condition such as this, it is normal for it to run rougher when electrical components are tuned on and it is normal for a car to run rough when the idle is to low.

My first instinct here is to clean the throttle body, fully warm the car, connect a scanner, and command the IAC (Idle Air Control) valve to move the idle high and low to see how it responds. 9 times out of ten, I have found this to fix this symptom.

Other things to consider:

Some more advanced possibilities include the MAF (Mass Air Flow) sensor, which is the primary sensor the PCM (Powertrain Control Module) uses to calculate air-fuel ratio. Although, I wouldn't go replacing the MAF without a thorough diagnosis from a certified technician.

I'd consider allowing a certified mechanic to diagnose your rough idle if you decide to get this fixed. You'll have a clear idea of what will need to be fixed after doing so.

Good luck!

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