Q: What is a "fouled" spark plug?
A: A spark plug is considered fouled when the insulator nose at the
firing tip becomes coated with a foreign substance such as fuel, oil or carbon.
This coating makes it easier for the voltage to follow along the insulator nose,
leach back down into the metal shell and ground out rather than
bridging the gap and firing normally.
Many factors can contribute to spark plug fouling. The air/fuel ratio may be
too rich as a result of incorrect carburetor adjustment or a poorly
performing fuel injection system. Worn piston rings or valve seals may
allow too much oil to leak into the combustion chamber, leading to oil
fouling. The ignition system may not be performing properly. Prolonged
idling or continuous low-speed driving may keep the spark plug from
reaching its optimum operating temperature. Using too cold a spark plug
can lead to the same problem. Finally, a dirty air cleaner can create a too-rich
condition which can lead to fouling.
Fuel, oil and carbon fouling can all be the result of different causes but,
once a spark plug is fouled, it will not provide adequate voltage to
the firing tip and that cylinder will not fire properly. In many cases,
the spark plug cannot be cleaned sufficiently to restore normal operation.
Therefore, it is recommended that a plug be replaced once it is fouled.
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