Spark Plug Overheating
Overheating is indicated by a white or gray center electrode insulator that also appears blistered. The increase in electrode gap will be considerably in excess of the normal 0.025 mm (.001 in.) per 3200 km (2000 miles) of operation. This suggests that a plug with a cooler heat range rating should be used. Over advanced ignition timing, detonation and cooling system malfunctions can also cause spark plug overheating (1).