I have a Chevrolet Malibu Classic 2.2L Ecotec, its well maintained with very low miles. It runs perfect in all situations except warm idle. Check engine light is not on so I assume no codes are set. When the car is cold or in park it idles perfect around 700-800 RPM. After the engine warms the idle drops to about 625 RPM. It idles very rough in drive, shakes the wheel and dash. Increasing idle with the peddle about 50 RPM smooths it out and idles runs perfect in drive. I checked plugs, and air cleaner, they look good. Does anyone have an idea where to look next?

 

Well my experience with when it idles rough and then smoothes out when engine rpm is brought up is a leaking valve. It may be running rough and actually misfiring but may not be counting enough misfires to set the DTC. Must be at least 20 counts per 100 to set DTC as pending and then must have 20 or more again to set CEL.

You may also have a dirty throttle body and or a small unmetered air leak. So you can clean throttle body and look for vacuum leaks like splits in hoses. Otherwise you may want to take it to dealer and have them inspect it since they'll be able to watch data