I got several codes indicating cylinder misfires. Engine running very rough. Turned engine off and back on and engines ran smooth but check engine remained on. Dealer said all spark plug way out of spec. (121K miles on car). Did the same thing after all plugs replaced. Tech advised of perhaps a deeper problem (cylinder 4 problem). The fact that when I restart the car it runs fine, gives me a feeling perhaps the problem is something electrical, like a sensor and not a mechanical issue. Any ideas?

 

Some initial thoughts:. RCM, Fuel pressure

 

You had several codes, if we do not know what they are we can offer no suggestions. The codes are the first things to study and correct.
A cylinder 4 code could be a coil problem. Swap it with another coil and see if the code moves with the coil.
If it was a plug problem then there would be a constant problem not an intermittent one. Do not trust a dealer their second language is lying with a straight face while they rob you blind.

 

Yea, stay away from dealers. ANY shop that has dealt with the M112 Mercedes engine knows far more about that car than the entire Chrysler corporate structure.

Additionally, without the code(s) that are setting off the check engine light, I have nothing to add.