In regards to my '89 420 SEL, I have had recent problems when attempting to start. The engine will turn over but will require an extended turn of the key. Once the car starts, it will idle roughly and shake for aproximately 20-30 seconds. Applying the gas lightly once the car starts seems to help the problem but is not a cure.
I have replaced the following parts in an attempt to correct the problem: spark plugs, wire set, distributor cap and rotor, fuel filter, O2 sensor, and air filters.
I have searched the archives and found numerous threads with this topic but I haven't found one that seemed to address this problem.