Jeep Cherokee (XJ): Diagnosis and testing

Jeep Cherokee (XJ) 1984 - 2001 Service Manual > Engine > 2.5L engine > Diagnosis and testing

Engine diagnosis-introduction

Engine diagnosis is helpful in determining the causes of malfunctions not detected and remedied by routine maintenance.

These malfunctions may be classified as either performance (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 possible 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 diagnosis 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.

Service diagnosis-performance

ENGINE PERFORMANCE DIAGNOSIS CHART

CONDITION	POSSIBLE CAUSES	CORRECTION
ENGINE WILL NOT CRANK	Weak or dead battery Corroded or loose battery connections Faulty starter or related circuit(s) Siezed accessory drive component Engine internal mechanical failure or hydro-static lock	Charge/Replace Battery. Refer to Group 8A, Battery, for correct procedures. Check charging system. Refer to Group 8C, Charging Systems, for correct procedures. Clean/tighten suspect battery/ starter connections Check starting system. Refer to Group 8B, Starting Systems, for correct diagnostics/procedures Remove accessory drive belt and attempt to start engine. If engine starts, repair/replace siezed component. Refer to Group 9, Engine, for correct diagnostics/procedures
ENGINE CRANKS BUT WILL NOT START	No spark No fuel Low or no engine compression	Check for spark. Refer to Group 8D, Ignition System, for correct procedures. Perform fuel pressure test, and if necessary, inspect fuel injector(s) and driver circuits. Refer to Group 14, Fuel System, for correct procedures. Perform cylinder compression pressure test. Refer to Group 9, Engine, for correct procedures.
ENGINE LOSS OF POWER	Worn or burned distributor rotor Worn distributor shaft Worn or incorrect gapped spark plugs Dirt or water in fuel system Faulty fuel pump Incorrect valve timing Blown cylinder head gasket Low compression Burned, warped, or pitted valves Plugged or restricted exhaust system Faulty ignition cables Faulty ignition coil	Install new distributor rotor Remove and repair distributor (Refer to group 8D, Ignition System Clean plugs and set gap. (Refer to group 8D, Ignition System) Clean system and replace fuel filter Install new fuel pump Correct valve timing Install new cylinder head gasket Test cylinder compression Install/Reface valves as necessary Install new parts as necessary Replace any cracked or shorted cables Test and replace, as necessary (Refer to Group 8D, ignition system)
ENGINE STALLS OR ROUGH IDLE	Carbon build-up on throttle plate Engine idle speed too low Worn or incorrectly gapped spark plugs Worn or burned distributor rotor Spark plug cables defective or crossed Faulty coil (Refer to group 8D, Ignition System) Intake manifold vacuum leak EGR valve leaking or stuck open	Remove throttle body and de-carbon. (Refer to Group 14 for correct procedures) Check Idle Air Control circuit. (Refer to Group 14, Fuel System) Replace or clean and re-gap spark plugs (Refer to group 8D, Ignition System) Install new distributor rotor Check for correct firing order or replace spark plug cables. (Refer to Group 8D, Ignition System for correct procedures.) Test and replace, if necessary Inspect intake manifold gasket and vacuum hoses. Replace if necessary (Refer to Group 11, Exhaust System & Intake Manifold) Test and replace, if necessary (Refer to group 25, Emission Control Systems)
ENGINE MISSES ON ACCELERATION	Worn or incorrectly gapped spark plugs Spark plug cables defective or crossed Dirt in fuel system Burned, warped or pitted valves Faulty coil	Replace spark plugs or clean and set gap. (Refer to group 8D, Ignition System) Check Idle Air Control circuit. (Refer to Group 14, Fuel System) Clean fuel system Install new valves Test and replace as necessary (refer to group 8D, Ignition System)

Service diagnosis-mechanical

ENGINE MECHANICAL DIAGNOSIS CHART

CONDITION	POSSIBLE CAUSES	CORRECTION
NOISY VALVES/LIFTERS	High or low oil level in crankcase Thin or diluted oil Low oil pressure Dirt in tappets/lash adjusters Bent push rod(s) Worn rocker arms and replace worn arms as needed Worn tappets/lash adjusters Worn valve guides replace as necessary Excessive runout of valve seats or valve faces	Check for correct oil level. Adjust oil level by draining or adding as needed Change oil (Refer to Engine Oil Service in this group) Check engine oil level. If ok, Perform oil pressure test. Refer to this group for engine oil pressure test/specifications Clean/replace hydraulic tappets/lash adjusters Install new push rods Inspect oil supply to rocker arms Install new hydraulic tappets/lash adjusters Inspect all valve guides and Grind valves and seats
CONNECTING ROD NOISE	Insufficient oil supply Low oil pressure Thin or diluted oil Excessive connecting rod bearing clearance Connecting rod journal out of round Misaligned connecting rods	Check engine oil level. (Refer to group 0, Lubrication and Maintenance) Check engine oil level. If ok, Perform oil pressure test. Refer to this group for engine oil pressure test/specifications Change oil to correct viscosity. Refer to this group for correct procedure/engine oil specifications Measure bearings for correct clearance with plasti-gage. Repair as necessary Replace crankshaft or grind journals Replace bent connecting rods
MAIN BEARING NOISE	Insufficient oil supply Low oil pressure Thin or diluted oil Excessive main bearing clearance Excessive end play Crankshaft main journal out of round or worn Loose flywheel or torque converter	Check engine oil level. (Refer to group 0, Lubrication and Maintenance) Check engine oil level. If ok, Perform oil pressure test. Refer to this group for engine oil pressure test/specifications Change oil to correct viscosity. Refer to this group for correct procedure/engine oil specifications Measure bearings for correct clearance. Repair as necessary Check crankshaft thrust bearing for excessive wear on flanges Grind journals or replace crankshaft Inspect crankshaft, flexplate/ flywheel and bolts for damage. Tighten to correct torque
LOW OIL PRESSURE	Low oil level Faulty oil pressure sending unit Clogged oil filter Worn oil pump Thin or diluted oil Excessive bearing clearance Oil pump relief valve stuck Oil pump suction tube loose, broken, bent or clogged Oil pump cover warped or cracked	Check oil level and fill if necessary Install new sending unit Install new oil filter Replace worn gears or oil pump assy Change oil to correct viscosity. Refer to this group for correct procedure/engine oil specifications Measure bearings for correct clearance Remove valve to inspect, clean and reinstall Inspect suction tube and clean or replace if necessary Install new oil pump
OIL LEAKS	Misaligned or deteriorated gaskets Loose fastener, broken or porous metal part Front or rear crankshaft oil seal leaking Leaking oil gallery plug or cup plug	Replace gasket Tighten, repair or replace the part Replace seal Remove and reseal threaded plug. Replace cup style plug
EXCESSIVE OIL CONSUMPTION OR SPARK PLUGS OIL FOULED	PCV System malfunction Defective valve stem seal(s) Worn or broken piston rings Scuffed pistons/cylinder walls Carbon in oil control ring groove Worn valve guides Piston rings fitted too tightly in grooves	Refer to group 25, Emission Control System for correct operation Repair or replace seal(s) Hone cylinder bores. Install new rings Hone cylinder bores and replace pistons as required Remove rings and de-carbon piston Inspect/replace valve guides as necessary Remove rings and check ring end gap and side clearance. Replace if necessary

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.

Cylinder compression pressure test

The results of a cylinder compression pressure test can be utilized to diagnose several engine malfunctions.

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) Disable the fuel system. (Refer to Group 14, Fuel System for the correct procedure) (5) Disconnect the ignition coil.

(6) Insert a compression pressure gauge and rotate the engine with the engine starter motor for three revolutions.

(7) Record the compression pressure on the 3rd revolution.

Continue the test for the remaining cylinders.

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 procedures outlined in Cylinder Compression Pressure Test. An engine cylinder head gasket leaking between adjacent cylinders will result in approximately a 50-70% reduction in compression pressure.

CYLINDER-TO-WATER JACKET LEAKAGE TEST

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.

Remove the radiator cap.

Start the engine and allow it to warm up until the engine thermostat opens.

If a large combustion/compression pressure leak exists, bubbles will be visible in the coolant.

If bubbles are not visible, install a radiator pressure tester and pressurize the coolant system.

If a cylinder is leaking combustion pressure into the water jacket, the tester pointer will pulsate with every combustion stroke of the cylinder.

Cylinder combustion pressure leakage test

The combustion pressure leakage test provides an accurate means for determining engine condition.

Combustion pressure leakage testing will detect:

Exhaust and intake valve leaks (improper seating).
Leaks between adjacent cylinders or into water jacket.
Any causes for combustion/compression pressure loss.

(1) Check the coolant level and fill as required. DO NOT install the radiator cap.

(2) Start and operate the engine until it attains normal operating temperature, then turn the engine OFF.

(3) Remove the spark plugs.

(4) Remove the oil filler cap.

(5) Remove the air cleaner.

(6) Calibrate the tester according to the manufacturer'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) recommended.

(7) Perform the test procedures on each cylinder according to the tester manufacturer's instructions.

While testing, listen for pressurized air escaping through the throttle body, tailpipe and 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 pressure, a minimum of 414 kPa (60 psi) should be maintained in the cylinder.

Refer to the Cylinder Combustion Pressure Leakage Test Diagnosis chart.

CYLINDER COMBUSTION PRESSURE LEAKAGE DIAGNOSIS CHART

CONDITION	POSSIBLE CAUSE	CORRECTION
AIR ESCAPES THROUGH THROTTLE BODY	Intake valve bent, burnt, or not seated properly	Inspect valve and valve seat. Reface or replace, as necessary
AIR ESCAPES THROUGH TAILPIPE	Exhaust valve bent, burnt, or not seated properly	Inspect valve and valve seat. Reface or replace, as necessary
AIR ESCAPES THROUGH RADIATOR	Head gasket leaking or cracked cylinder head or block	Remove cylinder head and inspect. Replace defective part
MORE THAN 50% LEAKAGE FROM ADJACENT CYLINDERS	Head gasket leaking or crack in cylinder head or block between adjacent cylinders	Remove cylinder head and inspect. Replace gasket, head, or block as necessary
MORE THAN 25% LEAKAGE AND AIR ESCAPES THROUGH OIL FILLER CAP OPENING ONLY	Stuck or broken piston rings; cracked piston; worn rings and/or cylinder wall	Inspect for broken rings or piston. Measure ring gap and cylinder diameter, taper and out-of-round. Replace defective part as necessary

Engine oil leak inspection

Begin with a thorough visual inspection of the engine, particularly at the area of the suspected leak.

If an oil leak source is not readily identifiable, the following steps should be followed: (1) Do not clean or degrease the engine at this time because some solvents may cause rubber to swell, temporarily stopping the leak.

(2) Add an oil soluble dye (use as recommended by manufacturer). Start the engine and let idle for approximately 15 minutes. Check the oil dipstick to make sure the dye is thoroughly mixed as indicated with a bright yellow color under a black light.

(3) Using a black light, inspect the entire engine for fluorescent dye, particularly at the suspected area of oil leak. If the oil leak is found and identified, repair per service manual instructions.

(4) If dye is not observed, drive the vehicle at various speeds for approximately 24km (15 miles), and repeat inspection.

(4) If the oil leak source is not positively identified at this time, proceed with the air leak detection test method.

Air Leak Detection Test Method

(1) Disconnect the breather cap to air cleaner hose at the breather cap end. Cap or plug breather cap nipple.

(2) Remove the PCV valve from the cylinder head cover. Cap or plug the PCV valve grommet.

(3) Attach an air hose with pressure gauge and regulator to the dipstick tube.

CAUTION: Do not subject the engine assembly to more than 20.6 kpa (3 PSI) of test pressure.

(4) Gradually apply air pressure from 1 psi to 2.5 psi maximum while applying soapy water at the suspected source. Adjust the regulator to the suitable test pressure that provide the best bubbles which will pinpoint the leak source. If the oil leak is detected and identified, repair per service manual procedures.

(5) If the leakage occurs at the rear oil seal area, refer to the section, Inspection for Rear Seal Area Leak.

(6) If no leaks are detected, turn off the air supply and remove the air hose and all plugs and caps.

Install the PCV valve and breather cap hose.

(7) Clean the oil off the suspect oil leak area using a suitable solvent. Drive the vehicle at various speeds approximately 24 km (15 miles). Inspect the engine for signs of an oil leak by using a black light.

INSPECTION FOR REAR SEAL AREA LEAKS

Since it is sometimes difficult to determine the source of an oil leak in the rear seal area of the engine, a more involved inspection is necessary. The following steps should be followed to help pinpoint the source of the leak.

If the leakage occurs at the crankshaft rear oil seal area: (1) Disconnect the battery.

(2) Raise the vehicle.

(3) Remove torque converter or clutch housing cover and inspect rear of block for evidence of oil.

Use a black light to check for the oil leak: (a) Circular spray pattern generally indicates seal leakage or crankshaft damage.

(b) Where leakage tends to run straight down, possible causes are a porous block, distributor seal, camshaft bore cup plugs oil galley pipe plugs, oil filter runoff, and main bearing cap to cylinder block mating surfaces.

(4) If no leaks are detected, pressurize the crankcase as outlined in the, Inspection (Engine oil Leaks in general) CAUTION: Do not exceed 20.6 kPa (3 psi).

(5) If the leak is not detected, very slowly turn the crankshaft and watch for leakage. If a leak is detected between the crankshaft and seal while slowly turning the crankshaft, it is possible the crankshaft seal surface is damaged. The seal area on the crankshaft could have minor nicks or scratches that can be polished out with emery cloth.

CAUTION: Use extreme caution when crankshaft polishing is necessary to remove minor nicks and scratches. The crankshaft seal flange is especially machined to complement the function of the rear oil seal.

(6) For bubbles that remain steady with shaft rotation, no further inspection can be done until disassembled.

Engine oil pressure

(1) Disconnect connector and remove oil pressure sending unit.

(2) Install Oil Pressure Line and Gauge Tool C-3292 or equivalent. Start engine and record pressure.

Refer to Oil Pressure in Engine Specifications for the correct pressures.

Description and operation

Engine DESCRIPTION The 2.5 liter (150 CID) four-cylinder engine is an In-line, lightweight, overhead valve engine. This engine is designed for unleaded fuel. The engine cylinder head has dual qu ...

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