1989 Mercedes Benz 420sel • 6 cylinder 2WD Automatic • 198,000 miles

My car is idling rough and occaisionally skips while driving
January 29, 2011.

Engine running rough or misfiring can be cause by the following:

1.Defective spark plugs
2.Inadequate spark/coil,defective spark plug wire.
3.Lack of compression
4.False air leakage.
5.Faulty fuel injectors.
6.Insufficient fuel pressure.
7.Contaminated fuel.
8.EGR valve that is leaking.
9.Oxygen sensors.
10.Throttle position sensor.
11.Manifold absolute pressure sensor
12.Mass Airflow Sensor
13.Idle air control valve

Note: If it doesn't apply disregard it-

Mar 16, 2011.
I've had the same problem and had a lot of work done, including replacing injectors, plugs, oxygen sensors, etc. The rough idle continued until I studied the repair manual and discovered the idle control valve. For a car this old, it was never replaced. I ran down to AutoZone and bought a new one with warranty and replace it. The rough idle immediately disappeared and the engine is so quite know at idle, I hardly notice it. This may not be your problem, but the fact that you have an 89 and have tried other things points to it Generally owners neglect this inexpensive part after many miles of driving.


George Wittenberg
Oct 29, 2011.