Grand Cherokee 4WD Laredo V8-4.7L (2002)
Fig.30 Pre-Ignition Damage
Pre-Ignition Damage
Pre-ignition damage is usually caused by excessive combustion chamber temperature. The center electrode dissolves first and the ground electrode
dissolves somewhat latter. Insulators appear relatively deposit free. Determine if the spark plug has the correct heat range rating for the engine.
Determine if ignition timing is over advanced or if other operating conditions are causing engine overheating. (The heat range rating refers to the
operating temperature of a particular type spark plug. Spark plugs are designed to operate within specific temperature ranges. This depends upon
the thickness and length of the center electrodes porcelain insulator.)
CAUTION: If the engine is equipped with copper core ground electrode, or platinum tipped spark plugs, they must be replaced with the same
type/number spark plug as the original. If another spark plug is substituted, pre-ignition will result.
Fig.31 Spark Plug Overheating
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 0.001 inch per 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.
CAUTION: If the engine is equipped with copper core ground electrode, or platinum tipped spark plugs, they must be replaced with the same
type/number spark plug as the original. If another spark plug is substituted, pre-ignition will result.
