Ford 2003 Mustang Workshop Manual page 930

4. Inspect for normal burning.
Check for light tan or gray deposits on the firing tip.
5. Inspect for pre-ignition, identified by melted electrodes and a possibly damaged insulator.
Metallic deposits on the insulator indicate engine damage. This may be caused by incorrect
ignition timing, wrong type of fuel or the unauthorized installation of a heli-coil insert in place of
the spark plug threads.
Install a new spark plug.
6. Inspect for overheating, identified by a white or light gray spots and with bluish-burnt
appearance of electrodes. This is caused by engine overheating, wrong type of fuel, loose spark
plugs, spark plugs with an incorrect heat range, low fuel pump pressure or incorrect ignition
timing.
Install a new spark plug.