PColumbo, try lightly prying under the problem coil pack with a thin piece of wood protecting the valve cover. After you remove the coil pack I would suspect the grounding stud can be removed and replaced. It may be left-hand thread. Best to have MB-trained techs work on your MB vehicles.

As mentioned, Bosch F8DC4 spark plugs (gapped per the Owner's Manual spec) work best in the M104 engines.

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Fred Hoelzle