Had a strange running issue that appeared as if the motor mounts failed. Under normal cruise, the CLK430 seemed fine but would have a rough idle at a stoplight or at operating temperature.

The number 5 ignition coil was arcing against the engine block, appearing as a bad spark plug wire. Turns out there was a small crack in the hard plastic seam where the boot connects. The arcing can be seen in the dark.
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Car is on original coils still and I just replaced the one bad one. I have the other 7 in a shopping cart ready to go. The car runs like new after the replacement by the way.

 

That’s good to hear you found this issue and I’ll check mine. Usually it’s the motor mounts, as most of us have replaced them. My CLK430 feels like a completely different car after replacing mine.