Grand Cherokee 2WD L6-4.0L VIN S (1997)
Fig. 4 Normal Operation And Cold (Carbon) Fouling
The few deposits present will be probably light tan or slightly gray in color with most grades of commercial gasoline (Fig. 4). The spark plug will
not show evidence of electrode burning. Gap growth will average less than approximately 0.025 mm (0.001 in) per 1,600 km (1,000 miles) of
operation. Spark plugs with normal wear can usually be cleaned, have the electrodes filed and gapped, and then reinstalled.
Some fuel refiners in several areas of the United States have introduced a manganese additive (MMT) for unleaded fuel. During combustion, fuel
with MMT may coat the entire tip of the spark plug with a rust colored deposit. The rust color deposits can be misdiagnosed as being caused by
coolant in the combustion chamber. Spark plug performance is not affected by MMT deposits.
Oil or Ash Encrusted
Oil Or Ash Encrusted
If one or more spark plugs are oil or oil ash encrusted, evaluate engine condition for the cause of oil entry into that particular combustion chamber.
Overheating
Spark Plug Inspection
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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 in per 1,000 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.
Premature Electrode Wear - Preignition Damage
