What is a fouled spark plug?

What is a fouled spark plug? It’s a plug with an electrode that’s covered in a substance that keeps it from firing correctly. This leads to misfiring, a rough idle, less power and hard starting.
Most of the time, this contamination comes from oil. If the piston oil rings aren’t sealing, oil makes its way into the combustion chamber, leaving a thick, white coating on the plug. If there’s too much fuel entering the cylinder, due to an injector problem or a clogged air filter, the fuel leaves a black coating on the plug. In extreme cases, excess oil or fuel can leave the plug looking wet.

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