Ford 2003 Mustang Workshop Manual page 2078

8. Inspect for overheating.
This can be identified by a white or light gray insulator with small black or gray brown
spots and with bluish-burnt appearance of the electrodes.
This may be caused by engine overheating, wrong type of fuel, loose spark plugs, too hot
a plug, low fuel pump pressure or incorrect ignition timing.
Install a new spark plug.
9. Inspect for fused spot deposits.
This can be identified by melted or spotty deposits resembling bubbles or blisters.
This may be caused by sudden acceleration.
Clean the spark plug.
Installation
1. Adjust the spark plug gap as necessary.