Jeep XJ. Manual — part 95
terminal at ignition coil end of cable. If resistance is
not within specifications as found in the Spark Plug
Cable Resistance chart, remove the cable from the
distributor cap. Connect the ohmmeter to the termi-
nal ends of the cable. If resistance is not within spec-
ifications
as
found
in
the
Spark
Plug
Cable
Resistance chart, replace the cable. Inspect the igni-
tion coil tower for cracks, burns or corrosion.
SPARK PLUG CONDITIONS
NORMAL OPERATING
The few deposits present on the spark plug will
probably be light tan or slightly gray in color. This is
evident with most grades of commercial gasoline
(Fig. 11). There will not be evidence of electrode
burning. Gap growth will not average more than
approximately 0.025 mm (.001 in) per 3200 km (2000
miles) of operation. Spark plugs that have normal
wear can usually be cleaned, have the electrodes
filed, have the gap set and then be installed.
Some fuel refiners in several areas of the United
States have introduced a manganese additive (MMT)
for unleaded fuel. During combustion, fuel with MMT
causes the entire tip of the spark plug to be coated
with a rust colored deposit. This rust color can be
misdiagnosed as being caused by coolant in the com-
bustion chamber. Spark plug performance may be
affected by MMT deposits.
COLD FOULING/CARBON FOULING
Cold fouling is sometimes referred to as carbon
fouling. The deposits that cause cold fouling are basi-
cally carbon (Fig. 11). A dry, black deposit on one or
two plugs in a set may be caused by sticking valves
or defective spark plug cables. Cold (carbon) fouling
of the entire set of spark plugs may be caused by a
clogged air cleaner element or repeated short operat-
ing times (short trips).
WET FOULING OR GAS FOULING
A spark plug coated with excessive wet fuel or oil
is wet fouled. In older engines, worn piston rings,
leaking valve guide seals or excessive cylinder wear
can cause wet fouling. In new or recently overhauled
engines, wet fouling may occur before break-in (nor-
mal oil control) is achieved. This condition can usu-
ally be resolved by cleaning and reinstalling the
fouled plugs.
OIL OR ASH ENCRUSTED
If one or more spark plugs are oil or oil ash
encrusted (Fig. 12), evaluate engine condition for the
cause of oil entry into that particular combustion
chamber.
ELECTRODE GAP BRIDGING
Electrode gap bridging may be traced to loose
deposits in the combustion chamber. These deposits
accumulate on the spark plugs during continuous
stop-and-go driving. When the engine is suddenly
subjected to a high torque load, deposits partially liq-
uefy and bridge the gap between electrodes (Fig. 13).
This short circuits the electrodes. Spark plugs with
electrode gap bridging can be cleaned using standard
procedures.
Fig. 11 Normal Operation and Cold (Carbon) Fouling
1 – NORMAL
2 – DRY BLACK DEPOSITS
3 – COLD (CARBON) FOULING
Fig. 12 Oil or Ash Encrusted
8D - 8
IGNITION SYSTEM
XJ
DIAGNOSIS AND TESTING (Continued)
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.
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.
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.
Fig. 13 Electrode Gap Bridging
1 – GROUND ELECTRODE
2 – DEPOSITS
3 – CENTER ELECTRODE
Fig. 14 Scavenger Deposits
1 – GROUND ELECTRODE COVERED WITH WHITE OR
YELLOW DEPOSITS
2 – CENTER ELECTRODE
Fig. 15 Chipped Electrode Insulator
1 – GROUND ELECTRODE
2 – CENTER ELECTRODE
3 – CHIPPED INSULATOR
XJ
IGNITION SYSTEM
8D - 9
DIAGNOSIS AND TESTING (Continued)
MECHANICAL DIAGNOSIS (KEY DIFFICULT TO
ROTATE)
Vehicles equipped with an automatic trans-
mission and a floor mounted shifter: a cable is
used to connect the interlock device in the steering
column assembly, to the transmission floor shift
lever. This interlock device is used to lock the trans-
mission shifter in the PARK position when the key
lock cylinder is rotated to the LOCKED or ACCES-
SORY position. The interlock device within the steer-
ing column is not serviceable. If repair is necessary,
the steering column assembly must be replaced.
Refer to Group 19, Steering for procedures.
If the ignition key is difficult to rotate to or from
the LOCK or ACCESSORY position, it may not be
the fault of the key cylinder or the steering column
components. The brake transmission shift interlock
cable may be out of adjustment. Refer to Brake
Transmission Shift Interlock Cable Adjustment in
Group 21, Transmissions for adjustment procedures.
Vehicles equipped with an automatic trans-
mission and a steering column mounted shifter:
an interlock device is located within the steering col-
umn. This interlock device is used to lock the trans-
mission shifter in the PARK position when the key
lock cylinder is in the LOCKED or ACCESSORY
position. If it is difficult to rotate the key to or from
the LOCK or ACCESSORY position, the interlock
device within the steering column may be defective.
This device is not serviceable. If repair is necessary,
the steering column assembly must be replaced.
Refer to Group 19, Steering for procedures.
Vehicles equipped with a manual transmis-
sion and a floor mounted shifter: on certain mod-
els, a lever is located on the steering column behind
the ignition key lock cylinder. The lever must be
manually operated to allow rotation of the ignition
key lock cylinder to the LOCK or ACCESSORY posi-
tion. If it is difficult to rotate the key to the LOCK or
ACCESSORY position, the lever mechanism may be
defective. This mechanism is not serviceable. If
repair is necessary, the steering column assembly
must be replaced. Refer to Group 19, Steering for
procedures.
On other models, the ignition key cylinder must be
depressed to allow it to be rotated into the LOCK or
ACCESSORY position. If it is difficult to rotate the
key to the LOCK or ACCESSORY position, the lock
mechanism within the steering column may be defec-
tive. This mechanism is not serviceable. If repair is
necessary, the steering column assembly must be
replaced. Refer to Group 19, Steering for procedures.
REMOVAL AND INSTALLATION
SPARK PLUG CABLES
REMOVAL
CAUTION: When
disconnecting
a
high
voltage
cable from a spark plug or from the distributor cap,
twist the rubber boot slightly (1/2 turn) to break it
loose (Fig. 18). Grasp the boot (not the cable) and
pull it off with a steady, even force.
INSTALLATION
Push the cable firmly onto the sparkplug.
SPARK PLUGS
PLUG REMOVAL
On the 4.0L 6–cylinder engine the spark plugs are
located below the coil rail assembly. To gain access to
Fig. 16 Preignition Damage
1 – GROUND ELECTRODE STARTING TO DISSOLVE
2 – CENTER ELECTRODE DISSOLVED
Fig. 17 Spark Plug Overheating
1 – BLISTERED WHITE OR GRAY COLORED INSULATOR
8D - 10
IGNITION SYSTEM
XJ
DIAGNOSIS AND TESTING (Continued)
any/all spark plug(s), refer to Ignition Coil-4.0L
Engine Removal/Installation.
(1) 2.5L 4–Cylinder Engine: Always remove spark
plug or ignition coil cables by grasping at the cable
boot (Fig. 18). Turn the cable boot 1/2 turn and pull
straight back in a steady motion. Never pull directly
on the cable. Internal damage to cable will result.
(2) Prior to removing the spark plug, spray com-
pressed air around the spark plug hole and the area
around the spark plug. This will help prevent foreign
material from entering the combustion chamber.
(3) Remove the spark plug using a quality socket
with a rubber or foam insert.
(4) Inspect the spark plug condition. Refer to
Spark Plugs Conditions.
PLUG CLEANING
The plugs may be cleaned using commercially
available spark plug cleaning equipment. After clean-
ing, file the center electrode flat with a small point
file or jewelers file before adjusting gap.
CAUTION: Never use a motorized wire wheel brush
to clean the spark plugs. Metallic deposits will
remain on the spark plug insulator and will cause
plug misfire.
PLUG GAP ADJUSTMENT
Check the spark plug gap with a gap gauge tool. If
the gap is not correct, adjust it by bending the
ground electrode (Fig. 19). Never attempt to adjust
the gap by bending the center electrode.
SPARK PLUG GAP
• 2.5L 4-Cylinder Engine Spark Plug Gap:.89 mm
(.035 in).
• 4.0L 6-Cylinder Engine Spark Plug Gap:.89 mm
(.035 in).
PLUG INSTALLATION
Always tighten spark plugs to the specified torque.
Over tightening can cause distortion. This may result
in a change in the spark plug gap, or a cracked por-
celain insulator.
When replacing the spark plug and ignition coil
cables, route the cables correctly and secure them in
the appropriate retainers. Failure to route the cables
properly can cause the radio to reproduce ignition
noise. It could cause cross ignition of the spark plugs,
or short circuit the cables to ground.
(1) Start the spark plug into the cylinder head by
hand to avoid cross threading.
(2) Tighten the spark plugs to 35-41 N·m (26-30 ft.
lbs.) torque.
(3) 2.5L 4–Cylinder Engine: Install spark plug
cables over spark plugs.
(4) 4.0L 6–Cylinder Engine: Install coil rail. Refer
to Ignition Coil-4.0L Engine Removal/Installation.
Fig. 18 Cable Removal
1 – SPARK PLUG CABLE AND BOOT
2 – SPARK PLUG BOOT PULLER
3 – TWIST AND PULL
4 – SPARK PLUG
Fig. 19 Setting Spark Plug Gap—Typical
1 – GAUGE
2 – SPARK PLUG
XJ
IGNITION SYSTEM
8D - 11
REMOVAL AND INSTALLATION (Continued)
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