Brown stain on spark plugs — What’s the Cause and Is It a Problem?

The brown stain on spark plugs is normal and doesn’t affect plug operation

The yellow-brown stain on spark plugs is a “corona” stain. If can be dark brown, light brown, tan or even yellow. The corona stain forms between the metal spark plug shell and the bottom of the spark plug wire or COP coil boot.

The brown stain on spark plugs is just dirt particles attracted to the plug by static electricity during firing

When a spark plug fires, the voltage flow creates static electricity. The static electricity can attract small oil or dirt particles present in the spark plug well or out in the open air. The attraction and heats bonds the particles to the porcelain.That’s what causes the  stain.

A corona stain on spark plugs is completely normal. It should not be mistaken for exhaust gas blow-by or broken seals inside the spark plug. A corona stain does not cause misfires. Nor does it indicate a flashover situation where firing voltage tracks down the outside of the porcelain.

Corona can sometimes be observed in dark conditions as a slight blue light around the base of the spark plug. It may may a hissing sounds as opposed to a snap sound caused by flashover.

©, 2022 Rick Muscoplat