Technician A says that brown or light tan stains on the insulator of the spark plug are normal and would not cause a misfire on their own. Technician B says the stains are caused by gases from the combustion chamber leaking past the shell and insulator.
Who is correct A or B?
It is technician A who is correct this time!
It is almost impossible for gases to pass between the inner shell and insulator because modern manufacturing methods that seal the two parts of the spark plug. Even if the combustion chamber reaches 2,000 PSI, the seal will hold. The stains the technicians are talking about are corona stains on the exposed ceramic body are normal as seen here or here. Corona stains are caused by the high amount of static electricity attracting particles of oil and dirt to the body of the plug. These stains were more common back in the day on pushrod engines where the plug was mounted to the side of the cylinder head and exposed to the underhood environment. Most modern engines put the spark plug in a deep well between the camshafts. But, corona stains can occur on high-mileage plugs if the spark plug tube seals leak or if dirt makes it past the boots for the ignition coils or wires.When you are taking the ASE G1 or even the A1, make sure you can read spark plugs and know what conditions are normal and abnormal. This video is sponsored by Autolite.