Why Ignition Coils Fail

[AlexB]

It is very simple, apparently. The rocker cover is some engines is sealed with a sealant which gets in the spark plug well and glues the coil boot to the metal. The technicians pull the coils from the wells in servicing by applying excessive force, thus damaging the boot. They also often save money on not using an expensive dielectric grease when installing the coils. This results in the surface discharge (a spark tracing along the surface of the spark plug insulator); then you have to pay for the new set of coils, fortunately supplied pre-lubricated.

See below a photo of the spark plug well with red sealant in it.