The brief
JiaZheng's X3 had a disturbing burnt odor drifting into the cabin from the front of the car, alarming enough to want it checked straight away. He brought it in. On these engines that smell usually points at the valve cover. The valve cover sits on top of the cylinder head and seals the top of the engine, keeping the oil in. On BMW it's made of plastic, and over years of heat cycling the plastic warps and the integrated gasket hardens, so the seal lets go and oil seeps out the edge. It runs down onto the exhaust manifold or other hot parts and burns off, which is the smell, and on a warm day it can drift into the cabin through the ventilation intake. A warped plastic valve cover doesn't reseal, so it needs replacing.
The diagnosis
A check with the engine warm found it, oil weeping from the valve cover seal, running down onto hot parts where it was burning off and making the smell. The rest of the engine's seals, the oil filter housing, the sump, checked out dry, so it was the valve cover. That's a replacement. The cover is plastic and warped and the gasket is built into it on these, you don't reseal it, so the call was a complete cover with a fresh gasket.
The work
The intake and the bits in the way came off, the old warped valve cover was removed, the mating face on the head cleaned up properly, and a new genuine BMW-spec valve cover fitted with its fresh gasket, every bolt torqued in sequence to spec so it seats evenly. The oil was topped to level and the engine run and checked warm for any weep. A road test confirmed the burnt smell was gone and the cover was dry.
The outcome
No more burnt oil smell in the cabin, the valve cover sealed and dry, and the oil staying where it should. The X3 went home with the leak sorted. A warped plastic valve cover only seeps worse and keeps cooking oil onto hot parts, so changing the cover and the gasket stopped the leak at its source, a clean engine and no more smell.
