The brief
The owner had been smelling burnt oil after each drive on the E220d, and when he popped the bonnet he could see fresh oil traces along the cylinder head. A workshop look-over found oil pooling in two of the plug wells, which is the giveaway: the valve cover seal had let go and oil was getting somewhere it shouldn't.
The valve cover is the lid over the top of the engine that seals the camshafts and oil in. On this engine it's a one-piece plastic part with the gaskets built in. When the plastic ages and cracks, or the gasket hardens, oil weeps out, runs down the hot head and burns off, which is the smell, and pools in the plug wells. Oil sitting in a plug well eventually creeps into the coil pack and kills it, so it's not something to leave.
The diagnosis
Off the engine, the inspection was conclusive. The valve cover itself had a hairline crack near one of its mounting bosses, and the integrated gasket had hardened well beyond reuse. A couple of the bolt seats had cracked too, which is why the seal had pulled loose in the first place, the cover couldn't hold itself down evenly any more.
That's a cover replacement, not just a new gasket. A cracked plastic cover with cracked bolt seats isn't something you re-seal and hope on.
The work
The old cover came off, the cylinder head face was cleaned back to bare metal, and a new Mercedes-spec valve cover went on with its fresh integrated seals and a new set of mounting bolts, torqued in the proper crossing pattern so it seats evenly.
The wiring, the intake and everything else that came off to get to it went back on.
A run afterwards, then a check of the head and the plug wells, confirmed everything was dry and there was no burning smell.
The outcome
Dry head, dry plug wells, and no smell after a road test.
The E220d went home with the top end sealed properly. Doing it as the complete cover, with fresh bolts, means the seal is held down the way it's meant to be, and the plug wells stay dry, so there's no slow drip working its way into a coil pack and turning a leak into a misfire down the line.
