Jeep 1996 Grand Cherokee Service Manual page 283

8D - 12 IGNITION SYSTEM
DIAGNOSIS AND TESTING (Continued)
SCAVENGER DEPOSITS
Fuel scavenger deposits may be either white or yellow
(Fig. 22). They may appear to be harmful, but this is a
normal condition caused by chemical additives in cer-
tain fuels. These additives are designed to change the
chemical nature of deposits and decrease spark plug
misfire tendencies. Notice that accumulation on the
ground electrode and shell area may be heavy, but the
deposits are easily removed. Spark plugs with scavenger
deposits can be considered normal in condition and can
be cleaned using standard procedures.
Fig. 22 Scavenger Deposits
CHIPPED ELECTRODE INSULATOR
A chipped electrode insulator usually results from
bending the center electrode while adjusting the
spark plug electrode gap. Under certain conditions,
severe detonation can also separate the insulator
from the center electrode (Fig. 23). Spark plugs with
this condition must be replaced.
Fig. 23 Chipped Electrode Insulator
PREIGNITION DAMAGE
Preignition damage is usually caused by excessive
combustion chamber temperature. The center electrode
dissolves first and the ground electrode dissolves some-
what latter (Fig. 24). 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.)
Fig. 24 Preignition Damage
SPARK PLUG OVERHEATING
Overheating is indicated by a white or gray center
electrode insulator that also appears blistered (Fig. 25).
The increase in electrode gap will be considerably in
excess of 0.001 inch per 1000 miles of operation. This
suggests that a plug with a cooler heat range rating
should be used. Over advanced ignition timing, detona-
tion and cooling system malfunctions can also cause
spark plug overheating.
Fig. 25 Spark Plug Overheating
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