Spark Plug: Description and Operation

SPARK PLUG CONDITIONS
Normal Service Definition
Normal service is assumed to be a mixture of idling, slow-speed, and high-speed driving. Occasionally, highway driving is needed for good spark plug assembly performance because it gives increased combustion heat that burns away any excess deposits of carbon or oxides that have built up from frequent idling or continual stop-and-go driving.

Spark Plug Boots
Spark plug assemblies are protected by an insulating boot made of special heat-resistant material which covers the spark plug assembly terminal and extends downward over a portion of the spark plug assembly insulator.

Spark Plug Flash-Over
Boots prevent flash-over which causes engine misfiring. Do not mistake corona discharge for flash-over or a shorted insulator. Corona is a steady blue light appearing around a spark plug assembly insulator, just above the shell crimp. It is the visible evidence of high-tension field, and has no effect on ignition performance. Usually it can be detected only in darkness.

Dark Rings Insulator
Spark plug discharge may repel dust particles, leaving a dark ring on the spark plug assembly insulator just above the shell. This ring is sometimes mistakenly regarded as evidence that combustion gases have blown out between shell and insulator.