The brief
Mr Lim's 520i had come in down on power, the engine running rough. Around the ten-year mark on a BMW this one is almost a rite of passage, the valve cover. He brought it in. The valve cover sits on top of the cylinder head and seals the oil in around the camshafts and the spark plug wells. On these it's moulded plastic, and after years of heat cycles the plastic warps and the gasket hardens, so oil seeps out, and the worst of it runs down into the spark plug wells where it pools around the coils and plugs. Oil where the spark is means a weak, intermittent spark, so the engine misfires, loses power and runs rough. A warped plastic cover doesn't flatten back out, so it gets replaced along with the gasket, and the oil-fouled plugs go with it.
The diagnosis
A look over the engine found the valve cover warped and leaking, with oil pooled in the spark plug wells around the coils, and the spark plugs oil-fouled, which is the misfire, the rough running and the power loss. Diagnostics confirmed misfires on the affected cylinders. The rest of the engine was sound. That's a valve cover and gasket replacement with fresh spark plugs, the coils cleaned and checked, rather than wiping the wells and waiting for the oil to come back.
The work
The old warped valve cover came off, the spark plug wells cleaned out, and a new genuine BMW-spec valve cover and gasket fitted, torqued down in sequence to the manual figures so it seals flat. The oil-fouled spark plugs were replaced with a fresh set, the ignition coils cleaned up and checked, and the PCV side of the cover checked while it was apart. The misfire codes were cleared and the adaptations reset. A road test confirmed a steady idle, no misfires, full power back, and no oil weeping from the cover.
The outcome
Full power back, a steady idle, no misfires, no warning light, no oil in the plug wells, and a dry valve cover. The 520i went home running the way it should. Oil in the spark plug wells only gets worse and takes the coils with it, so doing the cover, the gasket and the plugs together put the whole top end right in one go.
