Pieter Duvenhage is a regular at the garage. He’s an insurance agent and his BMW 318i is old enough to be outside the warranty period. Five months ago, Hennie replaced all the spark plugs to cure a misfire.
Two months later, the BMW was back and again misfiring. Hennie checked the spark plugs and found that number three showed signs of not firing. He fitted a new plug to that cylinder and, because the engine ran perfectly, he assumed the dirty plug was a dud and did not bother with further checking.
Last week, Pieter appeared in my office with the news that the BMW was misfiring again. He tried hard to be polite and succeeded, but I couldn’t blame him for being annoyed. Hennie tested the number-three plug and found that it could not be faulted. These cars, like so many new models, have a separate coil for each cylinder, so Hennie tested number-three coil and found it had an internal short.
This would explain the misfire, but why did it happen only when the plug got older? We suspect that the existence of a break in the insulation inside the coil would offer the flow of electrons a choice between going to earth via the break or earthing via the spark-plug gap. When the plug was new, the plug gap would offer the path of least resistance but, as the plug got older and the gap increased, the electrons would prefer to jump the break inside the coil.