The brief
Mr Alan's 640i came in misfiring, the engine stumbling and running rough. A misfire means a cylinder isn't burning cleanly, and our BMW specialist's eye went to the timing, because on this engine there's a known weak point there. The engine's timing keeps the camshafts and the crankshaft in step so the valves open at exactly the right moment. The timing gear is held by a screw, and on the early version of this engine that screw is aluminium, which is known to snap. When it does, the timing goes irregular, and that's exactly what causes the misfire. BMW recognised the problem and changed the screw to a metal one that doesn't break so easily, so the fix is to replace the broken aluminium screw with the improved metal version and reset the engine timing with the proper BMW tools.
The diagnosis
The misfire pointed at the timing, and the check confirmed it: the aluminium timing gear screw had broken, exactly as expected on this engine, which is why the timing was irregular and the engine misfiring. The rest of the engine was sound. That's a timing gear screw replacement, with the improved metal screw rather than another aluminium one, and the engine timing reset, rather than chasing a misfire that's really a timing fault.
The work
The valve cover came off, and as a matter of habit it got cleaned up while it was off, the broken aluminium timing gear screw removed and the improved metal screw fitted, and the engine timing variation reset to spec with the BMW special tools that the job needs. Everything was reassembled, the gaskets and seals renewed where disturbed, and the misfire codes cleared. 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 the timing back in step. Mr Alan got the 640i back running properly again. Replacing the broken screw with the improved metal version means this particular fault doesn't come back, and resetting the timing properly is what makes the misfire stay gone.
