Removal And Installation - Jeep 1997 Grand Cherokee Service Manual

GROUND ELECTRODE
ZG
DIAGNOSIS AND TESTING (Continued)
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. 19). Spark plugs with
this condition must be replaced.
Fig. 19 Chipped Electrode 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. 20). 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.
21). The increase in electrode gap will be consider-
ably in excess of 0.001 inch per 1000 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.
CENTER CHIPPED ELECTRODE INSULATOR BLISTERED GRAY COLORED INSULATOR GROUND ELECTRODE STARTING TO DISSOLVE

REMOVAL AND INSTALLATION

SPARK PLUG CABLES
CAUTION:
cable from a spark plug or from the distributor cap,
twist the rubber boot slightly (1/2 turn) to break it
loose (Fig. 22). Grasp the boot (not the cable) and
pull it off with a steady, even force.
Install cables into the proper engine cylinder firing
order (Fig. 23) or (Fig. 24).
When replacing the spark plug and coil cables,
route the cables correctly and secure in the proper
retainers. Failure to route the cables properly can
cause the radio to reproduce ignition noise. It could
also cause cross ignition of the plugs or short circuit
the cables to ground.
When installing new cables, make sure a positive
connection is made. A snap should be felt when a
good connection is made between the plug cable and
the distributor cap tower.
WHITE OR
IGNITION SYSTEM
Fig. 20 Preignition Damage
Fig. 21 Spark PlugOverheating
When disconnecting a high voltage
ELECTRODE CENTER DISSOLVED
8D - 11