Ford 2003 Mustang Workshop Manual page 2066

4. Inspect for gap bridged.
This can be identified by deposit build up closing the gap between electrodes.
This may be caused by oil or carbon fouling.
Clean the spark plug.
5. Inspect for oil fouling.
This can be identified by wet black deposits on the insulator shell bore electrodes.
This may be caused by excessive oil entering the combustion chamber through worn
rings and pistons, excessive clearance between valve guides and stems, or worn or
loose bearings. Correct the oil problem.
Install a new spark plug.
6. Inspect for carbon fouling.
This can be identified by black, dry, fluffy carbon deposits on the insulator tips, exposed
shell surfaces and electrodes.
This may be caused by too cold a plug, dirty air cleaner, damaged fuel pump, too rich a
fuel mixture or excessive idling.
Clean the spark plug.