The brief
Mr Wilson, a regular of ours, brought his E200 in misfiring, the engine stumbling and running rough. A misfire means a cylinder isn't burning cleanly, and left to run it dumps unburnt fuel into the exhaust and can cook the catalytic converter, so it gets sorted at the source. When we pulled the plugs, the giveaway was right there: they were contaminated with engine oil. The spark plugs sit down in tubes in the cylinder head, sealed off from the oil around the camshafts by plug seals. When those seals harden and fail, oil seeps down into the plug wells and onto the plugs and coils, and oil where the spark should be makes a weak, intermittent spark, so the engine misfires. Worn plug seals don't reseal, so they get replaced along with the oil-fouled plugs.
The diagnosis
Diagnostics confirmed the misfire, and the plugs came out fouled with engine oil. The expert's eye traced it to worn plug seals letting oil into the spark plug wells, which is the fouling and the misfire. The rest of the ignition and the engine were sound. That's a spark plug and plug seal replacement, the seals renewed so oil stays out of the wells and a fresh set of plugs to fire cleanly, rather than just wiping the plugs and waiting for the oil to come back.
The work
The oil-fouled spark plugs were removed, the plug wells cleaned out, and the worn plug seals replaced with new genuine Mercedes-spec seals so the wells stay dry. A fresh genuine Mercedes-spec set of spark plugs went in, gapped and torqued, the ignition coils cleaned and checked. The misfire codes were cleared and the adaptations reset so the engine relearns on a clean ignition system. A road test confirmed a steady idle, no misfire, the light staying off, and smooth power.
The outcome
A steady idle, clean pull through the revs, no misfire, no warning light, and no oil getting back into the plug wells. Mr Wilson got the E200 back running properly again. Oil in the spark plug wells only gets worse and takes the coils with it, so doing the plug seals and the plugs together fixed the misfire for good rather than papering over it.