Worn or damaged valve guides, worn or damaged piston rings, rich fuel mixture and a leaky head gasket can all be causes of spark plugs fouling. An improperly performing ignition system might also be a cause for spark plug fouling.
A spark plug can be fouled by fuel, oil and carbon. Worn or damaged pistons or valve guides allow oil into the combustion chamber, which drips onto the spark plugs. This causes oil-fouled spark plugs and is characterized by dark, wet, thick deposits on the tip of the plug. Carbon-fouled spark plugs are usually caused by a too-rich air/fuel mixture. This produces dark, fluffy deposits on the tip of the plug. A leaky head gasket can also be a cause of this type of fouling. Fuel fouling causes wet plugs and comes from unburned gasoline. Ignition problems can cause this type of fouling, as well as cold starts on a carbureted vehicle.
Spark plugs are designed to self-clean. The ceramic shell surrounding the tip of the plug gets hot during normal operation and cleans off any deposits from fuel, oil or carbon. High RPM operation and driving on the highway are particularly good for spark plug cleaning. Stop-and-go traffic situations and long periods of idling are not good for cleaning spark plugs.