Spark plug JEEP GRAND CHEROKEE 2002 WJ / 2.G User Guide

OPERATION
Although cylinder firing order is the same as 4.0L
Jeep engines of previous years, spark plug firing is
not. The 3 coils dual-fire the spark plugs on cylinders
1-6, 2-5 and/or 3-4. When one cylinder is being fired
(on compression stroke), the spark to the opposite
cylinder is being wasted (on exhaust stroke).
Battery voltage is supplied to the three ignition
coils from the ASD relay. The Powertrain Control
Module (PCM) opens and closes the ignition coil
ground circuit for ignition coil operation.
Base ignition timing is not adjustable.By con-
trolling the coil ground circuit, the PCM is able to set
the base timing and adjust the ignition timing
advance. This is done to meet changing engine oper-
ating conditions.
The ignition coil is not oil filled. The windings are
embedded in an epoxy compound. This provides heat
and vibration resistance that allows the ignition coil
to be mounted on the engine.
Because of coil design, spark plug cables (second-
ary cables) are not used. The cables are integral
within the coil rail.
REMOVAL
A one-piece coil rail assembly containing three
individual coils is used on the 4.0L engine (Fig. 13).
The coil rail must be replaced as one assembly. The
bottom of the coil is equipped with 6 individual rub-
ber boots (Fig. 13) to seal the 6 spark plugs to the
coil. Inside each rubber boot is a spring. The spring
is used for an electrical contact between the coil and
the top of the spark plug. These rubber boots and
springs are a permanent part of the coil and are not
serviced separately.
(1) Disconnect negative battery cable at battery.
(2) The coil is bolted directly to the cylinder head.
Remove 4 coil mounting bolts (Fig. 14).
(3) Carefully pry up coil assembly from spark
plugs. Do this by prying alternately at each end of
coil until rubber boots have disengaged from all
spark plugs. If boots will not release from spark
plugs, use a commercially available spark plug boot
removal tool. Twist and loosen a few boots from a few
spark plugs to help remove coil.
(4) After coil has cleared spark plugs, position coil
for access to primary electrical connector. Disconnect
connector from coil by pushing slide tab outwards to
right side of vehicle (Fig. 15). After slide tab has been
positioned outwards, push in on secondary release
lock (Fig. 15) on side of connector and pull connector
from coil.
(5) Remove coil from vehicle.
Fig. 13 Ignition Coil AssemblyÐ4.0L 6±Cylinder
Engine
1 - CYL. #6
2 - CYL. #5
3 - CYL. #4
4 - CYL. #3
5 - CYL. #2
6 - CYL. #1
7 - COILS (3)
8 - MOUNTING BOLTS (4)
9 - BOLT BASES (4)
10 - RUBBER BOOTS (6)
Fig. 14 Ignition Coil Rail LocationÐ4.0L 6±Cylinder
Engine
1 - COIL RAIL
2 - COIL MOUNTING BOLTS (4)
3 - COIL
4 - COIL ELECTRICAL CONNECTION
8I - 10 IGNITION CONTROLWJ
COIL RAIL (Continued)

INSTALLATION
(1) Connect engine harness connector to coil by
snapping into position. Move slide tab towards
engine (Fig. 15) for a positive lock.
(2) Position ignition coil rubber boots to all spark
plugs. Push down on coil assembly until bolt bases
have contacted cylinder head
(3) Install 4 coil mounting bolts. Loosely tighten 4
bolts just enough to allow bolt bases to contact cylin-
der head. Do a final tightening of each bolt in steps
down to 29 N´m (250 in. lbs.) torque. Do not apply
full torque to any bolt first.
(4) Connect negative battery cable to battery.
IGNITION COIL
DESCRIPTION
The 4.7L V±8 engine uses 8 dedicated, and individ-
ually fired coil (Fig. 16) for each spark plug. Each
coil is mounted directly to the top of each spark plug
(Fig. 17).
Fig. 15 Ignition Coil Electrical ConnectorÐ4.0L
6±Cylinder Engine
1 - REAR OF VALVE COVER
2 - COIL RAIL
3 - SLIDE TAB
4 - RELEASE LOCK
5 - COIL CONNECTOR
Fig. 16 Ignition CoilÐ4.7L Engine
1 - O-RING
2 - IGNITION COIL
3 - ELECTRICAL CONNECTOR
Fig. 17 Ignition Coil LocationÐ4.7L Engine
1 - IGNITION COIL
2 - COIL ELECTRICAL CONNECTOR
3 - COIL MOUNTING STUD/NUT
WJIGNITION CONTROL 8I - 11
COIL RAIL (Continued)

OPERATION
Battery voltage is supplied to the 8 ignition coils
from the ASD relay. The Powertrain Control Module
(PCM) opens and closes each ignition coil ground cir-
cuit at a determined time for ignition coil operation.
Base ignition timing is not adjustable.By con-
trolling the coil ground circuit, the PCM is able to set
the base timing and adjust the ignition timing
advance. This is done to meet changing engine oper-
ating conditions.
The ignition coil is not oil filled. The windings are
embedded in an epoxy compound. This provides heat
and vibration resistance that allows the ignition coil
to be mounted on the engine.
Because of coil design, spark plug cables (second-
ary cables) are not used.
REMOVAL
An individual ignition coil is used for each spark
plug (Fig. 18). The coil fits into machined holes in the
cylinder head. A mounting stud/nut secures each coil
to the top of the intake manifold (Fig. 19). The bot-
tom of the coil is equipped with a rubber boot to seal
the spark plug to the coil. Inside each rubber boot is
a spring. The spring is used for a mechanical contact
between the coil and the top of the spark plug. These
rubber boots and springs are a permanent part of the
coil and are not serviced separately. An o-ring (Fig.
18) is used to seal the coil at the opening into the cyl-
inder head.
(1) Depending on which coil is being removed, the
throttle body air intake tube or intake box may need
to be removed to gain access to coil.
(2) Disconnect electrical connector (Fig. 19) from
coil by pushing downward on release lock on top of
connector and pull connector from coil.
(3) Clean area at base of coil with compressed air
before removal.
(4) Remove coil mounting nut from mounting stud
(Fig. 19).
(5) Carefully pull up coil from cylinder head open-
ing with a slight twisting action.
(6) Remove coil from vehicle.
INSTALLATION
(1) Using compressed air, blow out any dirt or con-
taminants from around top of spark plug.
(2) Check condition of coil o-ring and replace as
necessary. To aid in coil installation, apply silicone to
coil o-ring.
(3) Position ignition coil into cylinder head opening
and push onto spark plug. Do this while guiding coil
base over mounting stud.
(4) Install mounting stud nut and tighten to 8 N´m
(70 in. lbs.) torque.(5) Connect electrical connector to coil by snapping
into position.
(6) If necessary, install throttle body air tube or
box.
Fig. 18 Ignition CoilÐ4.7L V±8
1 - O-RING
2 - IGNITION COIL
3 - ELECTRICAL CONNECTOR
Fig. 19 Ignition Coil
1 - IGNITION COIL
2 - COIL ELECTRICAL CONNECTOR
3 - COIL MOUNTING STUD/NUT
8I - 12 IGNITION CONTROLWJ
IGNITION COIL (Continued)

INSTALLATION
4.7L High-Output Engine Only
NOTE: The left sensor is identified by an identifica-
tion tag (LEFT). It is also identified by a larger bolt
head. The Powertrain Control Module (PCM) must
have and know the correct sensor left/right posi-
tions. Do not mix the sensor locations.
(1) Thoroughly clean knock sensor mounting holes.
(2) Install sensors (Fig. 22) into cylinder block.
NOTE: Over or under tightening the sensor mount-
ing bolts will affect knock sensor performance, pos-
sibly causing improper spark control. Always use
the specified torque when installing the knock sen-
sors. The torque for the knock senor bolt is rela-
tively light for an 8mm bolt.
NOTE: Note foam strip on bolt threads. This foam is
used only to retain the bolts to sensors for plant
assembly. It is not used as a sealant. Do not apply
any adhesive, sealant or thread locking compound
to these bolts.
(3) Install and tighten mounting bolts.Bolt
torque is critical.Refer to torque specification.
(4) Install intake manifold. Refer to Engine sec-
tion.
(5) Connect knock sensor pigtail wiring harness to
engine wiring harness near right / rear of intake
manifold (Fig. 23).
SPARK PLUG
DESCRIPTION
Both the 4.0L 6-cylinder and the 4.7L V-8 engine
use resistor type spark plugs. Standard 4.7L V-8
engines are equipped with ªfired in suppressor sealº
type spark plugs using a copper core ground elec-
trode. High-Output (H.O.) 4.7L V-8 engines are
equipped with unique plugs using a platinum rivet
located on the tip of the center electrode.
Because of the use of an aluminum cylinder head
on the 4.7L engine, spark plug torque is very critical.
To prevent possible pre-ignition and/or mechanical
engine damage, the correct type/heat range/number
spark plug must be used.Do not substitute any
other spark plug on the 4.7L H.O. engine. Seri-
ous engine damage may occur.
Plugs on both engines have resistance values rang-
ing from 6,000 to 20,000 ohms (when checked with at
least a 1000 volt spark plug tester).Do not use an
ohmmeter to check the resistance values of thespark plugs. Inaccurate readings will result.
Remove the spark plugs and examine them for
burned electrodes and fouled, cracked or broken por-
celain insulators. Keep plugs arranged in the order
in which they were removed from the engine. A sin-
gle plug displaying an abnormal condition indicates
that a problem exists in the corresponding cylinder.
Replace spark plugs at the intervals recommended in
Group O, Lubrication and Maintenance.
EXCEPT 4.7L H.O. ENGINE :Spark plugs that
have low mileage may be cleaned and reused if not
otherwise defective, carbon or oil fouled. Also refer to
Spark Plug Conditions.4.7L H.O. ENGINE :Never
clean spark plugs on the 4.7L H.O. engine. Damage
to the platinum rivet will result.
CAUTION: EXCEPT 4.7L H.O. ENGINE : 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.
H.O. Gap Adjustment:If equipped with the 4.7L
H.O. engine, do not use a wire-type gapping tool as
damage to the platinum rivet on the center electrode
may occur. Use a tapered-type gauge (Fig. 24).
DIAGNOSIS AND TESTING - 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. 24 PLUG GAP - 4.7L H.O.
1 - TAPER GAUGE
WJIGNITION CONTROL 8I - 15
KNOCK SENSOR (Continued)

(Fig. 25). 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 plugsexcept platinum tippedthat have
normal wear can usually be cleaned, have the elec-
trodes 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. 25). 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. 26), 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. 27).
This short circuits the electrodes. Spark plugs with
electrode gap bridging can be cleaned using standard
procedures.
SCAVENGER DEPOSITS
Fuel scavenger deposits may be either white or yel-
low (Fig. 28). They may appear to be harmful, but
this is a normal condition caused by chemical addi-
tives in certain fuels. These additives are designed to
change the chemical nature of deposits and decrease
spark plug misfire tendencies. Notice that accumula-
tion on the ground electrode and shell area may be
heavy, but the deposits are easily removed. Spark
plugs with scavenger deposits can be considered nor-
mal in condition and can be cleaned using standard
procedures.
Fig. 25 NORMAL OPERATION AND COLD (CARBON)
FOULING
1 - NORMAL
2 - DRY BLACK DEPOSITS
3 - COLD (CARBON) FOULING
Fig. 26 OIL OR ASH ENCRUSTED
8I - 16 IGNITION CONTROLWJ
SPARK PLUG (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. 29). Spark plugs with
this condition must be replaced.
PRE-IGNITION DAMAGE
Pre-ignition damage is usually caused by excessive
combustion chamber temperature. The center elec-
trode dissolves first and the ground electrode dis-
solves somewhat latter (Fig. 30). 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.)
CAUTION: If the engine is equipped with copper
core ground electrode, or platinum tipped spark
plugs, they must be replaced with the same type/
number spark plug as the original. If another spark
plug is substituted, pre-ignition will result.
Fig. 27 ELECTRODE GAP BRIDGING
1 - GROUND ELECTRODE
2 - DEPOSITS
3 - CENTER ELECTRODE
Fig. 28 SCAVENGER DEPOSITS
1 - GROUND ELECTRODE COVERED WITH WHITE OR
YELLOW DEPOSITS
2 - CENTER ELECTRODE
Fig. 29 CHIPPED ELECTRODE INSULATOR
1 - GROUND ELECTRODE
2 - CENTER ELECTRODE
3 - CHIPPED INSULATOR
Fig. 30 PRE-IGNITION DAMAGE
1 - GROUND ELECTRODE STARTING TO DISSOLVE
2 - CENTER ELECTRODE DISSOLVED
WJIGNITION CONTROL 8I - 17
SPARK PLUG (Continued)

SPARK PLUG OVERHEATING
Overheating is indicated by a white or gray center
electrode insulator that also appears blistered (Fig.
31). 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.
CAUTION: If the engine is equipped with copper
core ground electrode, or platinum tipped spark
plugs, they must be replaced with the same type/
number spark plug as the original. If another spark
plug is substituted, pre-ignition will result.
REMOVAL
CAUTION: If equipped with a 4.7L H.O. (High-Out-
put) engine, never substitute the original platinum
tipped spark plug with a different part number. Seri-
ous engine damage may result.
On the 4.0L 6±cylinder engine, the spark plugs are
located below the coil rail assembly. On the 4.7L V±8
engine, each individual spark plug is located under
each ignition coil.
(1) 4.0L 6±Cylinder Engine: Prior to removing
spark plug, spray compressed air around spark plug
hole and area around spark plug. This will help pre-
vent foreign material from entering combustion
chamber.
(2) 4.7L V±8 Engine: Prior to removing spark plug,
spray compressed air around base of ignition coil at
cylinder head. This will help prevent foreign material
from entering combustion chamber.
(3) On the 4.0L engine the coil rail assembly must
be removed to gain access to any/all spark plug.
Refer to Ignition Coil Removal/Installation. On the4.7L V-8 engine each individual ignition coil must be
removed to gain access to each spark plug. Refer to
Ignition Coil Removal/Installation.
(4) Remove spark plug from cylinder head using a
quality socket with a rubber or foam insert. If
equipped with a 4.7L V-8 engine, also check condition
of coil o-ring and replace as necessary.
(5) Inspect spark plug condition. Refer to Spark
Plug Conditions.
CLEANING
Except 4.7L H.O. Engine:The plugs may be
cleaned using commercially available spark plug
cleaning equipment. After cleaning, file center elec-
trode flat with a small point file or jewelers file
before adjusting gap.
CAUTION: Never use a motorized wire wheel brush
to clean spark plugs. Metallic deposits will remain
on spark plug insulator and will cause plug misfire.
4.7L H.O. Engine:Never clean spark plugs on the
4.7L H.O. engine. Damage to the platinum rivet on
the center electrode will result.
INSTALLATION
CAUTION: The standard 4.7L V-8 engine is
equipped with copper core ground electrode spark
plugs. They must be replaced with the same type/
number spark plug as the original. If another spark
plug is substituted, pre-ignition will result.
CAUTION: If equipped with a 4.7L H.O. (High-Out-
put) engine, never substitute the original platinum
tipped spark plug with a different type/part number.
Serious engine damage may result.
Special care should be taken when installing spark
plugs into cylinder head spark plug wells. Be sure
plugs do not drop into plug wells as ground straps
may be bent resulting in a change in plug gap, or
electrodes can be damaged.
Always tighten spark plugs to specified torque. Over
tightening can cause distortion resulting in a change
in spark plug gap or a cracked porcelain insulator.
(1) Start spark plug into cylinder head by hand to
avoid cross threading.
(2) 4.0L 6±Cylinder Engine: Tighten spark plugs to
35-41 N´m (26-30 ft. lbs.) torque.
(3) 4.7L V±8 Engine: Tighten spark plugs to 27
N´m (20 ft. lbs.) torque.
(4)
4.7L V±8 Engine: Before installing coil(s), check
condition of coil o-ring and replace as necessary. To aid
in coil installation, apply silicone to coil o-ring.
(5) Install ignition coil(s). Refer to Ignition Coil
Removal/Installation.
Fig. 31 SPARK PLUG OVERHEATING
1 - BLISTERED WHITE OR GRAY COLORED INSULATOR
8I - 18 IGNITION CONTROLWJ
SPARK PLUG (Continued)

CONDITION POSSIBLE CAUSES CORRECTION
3. Worn or incorrect gapped spark
plugs3. Clean plugs and set gap. (Refer to
8 - ELECTRICAL/IGNITION
CONTROL/SPARK PLUG -
CLEANING).
4. Dirt or water in fuel system 4. Clean system and replace fuel
filter
5. Faulty fuel pump 5. Install new fuel pump
6. Incorrect valve timing 6. Correct valve timing
7. Blown cylinder head gasket 7. Install new cylinder head gasket
8. Low compression 8. Test cylinder compression. (Refer
to 9 - ENGINE - DIAGNOSIS AND
TESTING).
9. Burned, warped, or pitted valves 9. Install/Reface valves as necessary
10. Plugged or restricted exhaust
system10. Install new parts as necessary
11. Faulty ignition coil rail 11. Test and replace, as necessary.
(Refer to 8 - ELECTRICAL/IGNITION
CONTROL/COIL RAIL - REMOVAL).
ENGINE STALLS OR ROUGH
IDLE1. Carbon build-up on throttle plate 1. Remove throttle body and
de-carbon. (Refer to 14 - FUEL
SYSTEM/FUEL INJECTION/
THROTTLE BODY - REMOVAL) for
correct procedure.
2. Engine idle speed too low 2. Check Idle Air Control circuit.
3. Worn or incorrectly gapped spark
plugs3. Replace or clean and re-gap spark
plugs. (Refer to 8 - ELECTRICAL/
IGNITION CONTROL/SPARK PLUG
- CLEANING)
4. Faulty coil rail 4. Test and replace, if necessary.
(Refer to 8 - ELECTRICAL/IGNITION
CONTROL/COIL RAIL - REMOVAL)
5. Intake manifold vacuum leak 5. Inspect intake manifold gasket and
vacuum hoses. Replace if necessary.
(Refer to 9 - ENGINE/MANIFOLDS/
INTAKE MANIFOLD - DIAGNOSIS
AND TESTING).
ENGINE MISSES ON
ACCELERATION1. Worn or incorrectly gapped spark
plugs1. Replace spark plugs or clean and
set gap.
2. Spark plug cables defective or
crossed2. Replace spark plug cables.
3. Dirt in fuel system 3. Clean fuel system
4. Burned, warped or pitted valves 4. Install new valves
5. Faulty coil rail 5. Test and replace as necessary.
(Refer to 8 - ELECTRICAL/IGNITION
CONTROL/COIL RAIL - REMOVAL)
WJENGINE - 4.0L 9 - 5
ENGINE - 4.0L (Continued)

CONDITION POSSIBLE CAUSES CORRECTION
3. Thin or diluted oil 3. Change oil to correct viscosity.
4. Excessive main bearing clearance 4. Measure bearings for correct
clearance. Repair as necessary
5. Excessive end play 5. Check crankshaft thrust bearing for
excessive wear on flanges
6. Crankshaft main journal out of
round or worn6. Grind journals or replace
crankshaft
7. Loose flywheel or torque converter 7. Inspect crankshaft, flexplate/
flywheel and bolts for damage.
Tighten to correct torque
LOW OIL PRESSURE 1. Low oil level 1. Check oil level and fill if necessary
2. Faulty oil pressure sending unit 2. Install new sending unit
3. Clogged oil filter 3. Install new oil filter
4. Worn oil pump 4. Replace oil pump assembly.
5. Thin or diluted oil 5. Change oil to correct viscosity.
6. Excessive bearing clearance 6. Measure bearings for correct
clearance
7. Oil pump relief valve stuck 7. Remove valve to inspect, clean
and reinstall
8. Oil pump suction tube loose,
broken, bent or clogged8. Inspect suction tube and clean or
replace if necessary
9. Oil pump cover warped or cracked 9. Install new oil pump
OIL LEAKS 1. Misaligned or deteriorated gaskets 1. Replace gasket
2. Loose fastener, broken or porous
metal part2. Tighten, repair or replace the part
3. Front or rear crankshaft oil seal
leaking3. Replace seal
4. Leaking oil gallery plug or cup
plug4. Remove and reseal threaded plug.
Replace cup style plug
EXCESSIVE OIL
CONSUMPTION OR SPARK
PLUGS OIL FOULED1. CCV System malfunction 1. (Refer to 25 - EMISSIONS
CONTROL/EVAPORATIVE
EMISSIONS - DESCRIPTION) for
correct operation
2. Defective valve stem seal(s) 2. Repair or replace seal(s)
3. Worn or broken piston rings 3. Hone cylinder bores. Install new
rings
4. Scuffed pistons/cylinder walls 4. Hone cylinder bores and replace
pistons as required
5. Carbon in oil control ring groove 5. Remove rings and de-carbon
piston
6. Worn valve guides 6. Inspect/replace valve guides as
necessary
7. Piston rings fitted too tightly in
grooves7. Remove rings and check ring end
gap and side clearance. Replace if
necessary
WJENGINE - 4.0L 9 - 7
ENGINE - 4.0L (Continued)

DIAGNOSIS AND TESTINGÐCYLINDER
COMPRESSION PRESSURE
The results of a cylinder compression pressure test
can be utilized to diagnose several engine malfunc-
tions.
Ensure the battery is completely charged and the
engine starter motor is in good operating condition.
Otherwise, the indicated compression pressures may
not be valid for diagnosis purposes.
(1) Clean the spark plug recesses with compressed
air.
(2) Remove the spark plugs (Refer to 8 - ELEC-
TRICAL/IGNITION CONTROL/SPARK PLUG -
REMOVAL).
(3) Secure the throttle in the wide-open position.
(4) Disconnect the ignition coil.
(5) Insert a compression pressure gauge and rotate
the engine with the engine starter motor for three
revolutions.
(6) Record the compression pressure on the third
revolution. Continue the test for the remaining cylin-
ders.
(Refer to 9 - ENGINE - SPECIFICATIONS) for the
correct engine compression pressures.
DIAGNOSIS AND TESTING - CYLINDER
COMBUSTION PRESSURE LEAKAGE
The combustion pressure leakage test provides an
accurate means for determining engine condition.
Combustion pressure leakage testing will detect:
²Exhaust and intake valve leaks (improper seat-
ing)
²Leaks between adjacent cylinders or into water
jacket²Any causes for combustion/compression pressure
loss
WARNING: DO NOT REMOVE THE RADIATOR CAP
WITH THE SYSTEM HOT AND UNDER PRESSURE.
SERIOUS BURNS FROM HOT COOLANT CAN
OCCUR.
Check the coolant level and fill as required. DO
NOT install the radiator cap.
Start and operate the engine until it attains nor-
mal operating temperature, then turn OFF the
engine.
Remove the spark plugs.
Remove the oil filler cap.
Remove the air cleaner.
Calibrate the tester according to the manufactur-
er's instructions. The shop air source for testing
should maintain 483 kPa (70 psi) minimum, 1,379
kPa (200 psi) maximum and 552 kPa (80 psi) recom-
mended.
Perform the test procedure on each cylinder accord-
ing to the tester manufacturer's instructions. While
testing, listen for pressurized air escaping through
the throttle body, tailpipe or oil filler cap opening.
Check for bubbles in the radiator coolant.
All gauge pressure indications should be equal,
with no more than 25% leakage.
FOR EXAMPLE:At 552 kPa (80 psi) input pres-
sure, a minimum of 414 kPa (60 psi) should be main-
tained in the cylinder CYLINDER COMBUSTION
PRESSURE LEAKAGE DIAGNOSIS CHART .
CYLINDER COMBUSTION PRESSURE LEAKAGE DIAGNOSIS CHART
CONDITION POSSIBLE CAUSE CORRECTION
AIR ESCAPES THROUGH
THROTTLE BODYIntake valve bent, burnt, or not
seated properlyInspect valve and valve seat.
Reface or replace, as necessary
AIR ESCAPES THROUGH
TAILPIPEExhaust valve bent, burnt, or not
seated properlyInspect valve and valve seat.
Reface or replace, as necessary
AIR ESCAPES THROUGH
RADIATORHead gasket leaking or cracked
cylinder head or blockRemove cylinder head and inspect.
Replace defective part
MORE THAN 50% LEAKAGE
FROM ADJACENT CYLINDERSHead gasket leaking or crack in
cylinder head or block between
adjacent cylindersRemove cylinder head and inspect.
Replace gasket, head, or block as
necessary
MORE THAN 25% LEAKAGE AND
AIR ESCAPES THROUGH OIL
FILLER CAP OPENING ONLYStuck or broken piston rings;
cracked piston; worn rings and/or
cylinder wallInspect for broken rings or piston.
Measure ring gap and cylinder
diameter, taper and out-of-round.
Replace defective part as necessary
9 - 8 ENGINE - 4.0LWJ
ENGINE - 4.0L (Continued)