Hello i have a 1993 Mercedes Benz 400sel. i have changed the distributor caps, rotors, and spark plugs/wires. it is only running on 4 cylinders right now, and i don't know why. There is no spark on all 4 spark plug wires that connect to the left distributor cap. but there is a spark on the 5th wire. what could be the problem? ignition coil? engine harness?