Ford 2003 Mustang Workshop Manual page 2067

7. Inspect for normal burning.
This can be identified by light tan or gray deposits on the firing tip.
8. Inspect for pre-ignition.
This can be identified by melted electrodes and possibly a blistered insulator. Metallic
deposits on the insulator indicate engine damage.
Install a new spark plug.
9. Inspect for overheating.
This can be identified by a white or light gray insulator with small black or gray brown
spots and a bluish-burnt appearance of the electrodes.
This may be caused by engine overheating, the wrong type of fuel, loose spark plugs, too
hot a plug, low fuel pump pressure or incorrect ignition timing.
Install a new spark plug.