Jeep 1996 Grand Cherokee Service Manual page 945

9 - 8 ENGINE
DIAGNOSIS AND TESTING
CYLINDER COMBUSTION PRESSURE
LEAKAGE TEST . . . . . . . . . . . . . . . . . . . . . . . . 9
CYLINDER COMPRESSION PRESSURE TEST . . 8
ENGINE CYLINDER HEAD GASKET FAILURE
DIAGNOSIS . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
ENGINE OIL PRESSURE . . . . . . . . . . . . . . . . . . 11
ENGINE—LUBRICATION . . . . . . . . . . . . . . . . . . 14
DIAGNOSIS AND TESTING
GENERAL INFORMATION
Engine diagnosis is helpful in determining the
causes of malfunctions not detected and remedied by
routine tune-ups.
These malfunctions may be classified as either per-
formance (e.g., engine idles rough and stalls) or
mechanical (e.g., a strange noise).
Refer to the Service Diagnosis—Performance chart
and the Service Diagnosis—Mechanical chart for pos-
sible causes and corrections of malfunctions. Refer to
Group 14, Fuel System for the fuel system diagnosis.
Additional tests and diagnostic procedures may be
necessary for specific engine malfunctions that can
not be isolated with the Service Diagnosis charts.
Information concerning additional tests and diagno-
sis is provided within the following diagnosis:
• Cylinder Compression Pressure Test.
• Cylinder Combustion Pressure Leakage Test.
• Engine Cylinder Head Gasket Failure Diagnosis.
• Intake Manifold Leakage Diagnosis.
INTAKE MANIFOLD LEAKAGE DIAGNOSIS
An intake manifold air leak is characterized by
lower than normal manifold vacuum. Also, one or
more cylinders may not be functioning.
WARNING: USE EXTREME CAUTION WHEN THE
ENGINE IS OPERATING. DO NOT STAND IN A
DIRECT LINE WITH THE FAN. DO NOT PUT YOUR
HANDS NEAR THE PULLEYS, BELTS OR THE FAN.
DO NOT WEAR LOOSE CLOTHING.
(1) Start the engine.
(2) Spray a small stream of water at the suspected
leak area.
(3) If a change in RPM is observed the area of the
suspected leak has been found.
(4) Repair as required.
ENGINE DIAGNOSIS
INDEX
page
ENGINE—MECHANICAL . . . . . . . . . . . . . . . . . . 13
ENGINE—PERFORMANCE . . . . . . . . . . . . . . . . 12
GENERAL INFORMATION . . . . . . . . . . . . . . . . . . 8
HYDRAULIC TAPPETS . . . . . . . . . . . . . . . . . . . . 10
INSPECTION (ENGINE OIL LEAKS IN
GENERAL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
INTAKE MANIFOLD LEAKAGE DIAGNOSIS . . . . . 8
CYLINDER COMPRESSION PRESSURE TEST
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.
(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 3rd
revolution. Continue the test for the remaining cylin-
ders.
Refer to Engine Specifications for the correct
engine compression pressures.
ENGINE CYLINDER HEAD GASKET FAILURE
DIAGNOSIS
A leaking engine cylinder head gasket usually
results in loss of power, loss of coolant and engine
misfiring.
An engine cylinder head gasket leak can be located
between adjacent cylinders or between a cylinder and
the adjacent water jacket.
• An engine cylinder head gasket leaking between
adjacent cylinders is indicated by a loss of power
and/or engine misfire.
• An engine cylinder head gasket leaking between
a cylinder and an adjacent water jacket is indicated
by coolant foaming or overheating and loss of coolant.
CYLINDER-TO-CYLINDER LEAKAGE TEST
To determine if an engine cylinder head gasket is
leaking between adjacent cylinders; follow the proce-
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