I had a rough idle start over the weekend where the motor was really rocking. I took to my mechanic friend and they found the plug wires to be original (120,000 miles) and the plugs had started fouling out. The rotor button was burnt up. So, he replaced the wires, plugs, dist. cap, and rotor button. The car runs now upon acceleration like a it has never before. It is much smother, but at idle, it is rough. Does anyone know what might be the problem? I have noticed that the tranny seems to shift harder now. Maybe it is because it has more power because one of the plugs was not even firing before. Any suggestions of what to check? I have noticed that there is a slight chatter of what seems to be the lifters. I have had a timing chain job done about 10k ago. Could it be the injectors? If injectors, would I notice the rpms surging? that seems to be smooth.

If you use the search feature, there is lots of info at this site about rough idling 420SEL's. Also check the rough idle on 560SEL's the engines have similar principles to the 420SEL's engine.

There are about 10 different things that could be causing the rough idle.

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1989 420 SEL

You may want to check to the right of the air/fuel mixture housing where the "Air Injection" shut off valve gets it's vacuum. This hose blows off from time to time causing an immediate idle quality issue. A new 180 degree pre-formed hose, and or a small tie wrap should secure it for quite some time.