BMW E34 ignition coil and spark plug fault: one cylinder dead
E34 jerks or one cylinder does not fire - how do I find the faulty ignition coil?
Unplug the coils one at a time: if the running does not change, that coil is faulty. Another way is to swap a new coil in place of an old one, one at a time, until the fault goes away. A coil may first only jerk and cut out more and more before it stops completely.
Why are the spark plugs oily or is there oil in the plug well?
Usually the oil comes through the ageing valve cover gasket into the plug well. The oil can discharge the spark into the cover, so the cylinder does not fire even though the plugs are new. The repair is replacing the gaskets; also check that the valve cover has not loosened. Clean the wells and plugs.
How do I check the ignition components?
The M50 engine has no plug wires; the coils sit directly on the plugs, and hairline cracks can develop in their insulators from heat. If the coil fault appears only when warm, the fault may be in the insulation of the coil winding. The resistance of the plug cap can be measured with a multimeter. If the resistance is the same cold and warm, the fault is probably elsewhere.