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XJ
DIAGNOSIS AND TESTING (Continued)
Fig. 13 Electrode Gap Bridging
1 – GROUND ELECTRODE
2 – DEPOSITS
3 – CENTER ELECTRODE
SCAVENGER DEPOSITS
Fuel scavenger deposits may be either white or yel-
low (Fig. 14). They may appear to be harmful, but this
is a normal condition caused by chemical additives in
certain 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 condi-
tion and can be cleaned using standard procedures.
Fig. 14 Scavenger Deposits
1 – GROUND ELECTRODE COVERED WITH WHITE OR
YELLOW DEPOSITS
2 – CENTER ELECTRODE
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. 15). Spark plugs with
this condition must be replaced.
Fig. 15 Chipped Electrode Insulator
1 – GROUND ELECTRODE
2 – CENTER ELECTRODE
3 – CHIPPED INSULATOR
PREIGNITION DAMAGE
Preignition damage is usually caused by excessive
combustion chamber temperature. The center elec-
trode dissolves first and the ground electrode dis-
solves somewhat latter (Fig. 16). 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 over-
heating. (The heat range rating refers to the operat-
ing temperature of a particular type spark plug.
Spark plugs are designed to operate within specific
temperature ranges. This depends upon the thick-
ness and length of the center electrodes porcelain
insulator.)
SPARK PLUG OVERHEATING
Overheating is indicated by a white or gray center
electrode insulator that also appears blistered (Fig.
17). The increase in electrode gap will be consider-
ably in excess of 0.001 inch per 2000 miles of opera-
tion. 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.
IGNITION SWITCH AND KEY LOCK CYLINDER
ELECTRICAL DIAGNOSIS
For ignition switch electrical schematics, refer to
Ignition Switch in Group 8W, Wiring Diagrams.
IGNITION SYSTEM
8D - 9
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