Ford 2003 Mustang Workshop Manual page 2077

Install a new spark plug.
5. Inspect for carbon fouling.
This can be identified by black, dry fluffy carbon deposits on 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.
6. Inspect for normal burning.
This can be identified by light tan or gray deposits on the firing tip.
7. Inspect for pre-ignition.
This can be identified by melted electrodes and possibly a blistered insulator. Metallic
deposits on insulator indicate engine damage.
Install a new spark plug.