White or Gray - Burned or Blistered Insulator
Spark Plug Overheating:

Symptoms: White or gray center electrode insulator that also appears blistered. The increase in electrode gap will be considerably in excess of 0.001 in per 1000 miles of operation.
Possible Causes:
- Plug(s) with wrong heat range.
- Over advanced ignition timing.
- Detonation
- Cooling system malfunction
- Check for lean air-fuel mixture
- Check the torque value of the plugs to ensure good plug-engine seat contact