© 2012 delmar, cengage learning diagnosing engine mechanical problems chapter 49
TRANSCRIPT
© 2012 Delmar, Cengage Learning
Objectives• Use engine diagnostic tools and equipment
safely and properly• Diagnose engine and related problems prior to
repair
© 2012 Delmar, Cengage Learning
Introduction• Major diagnosis areas
– Oil consumption
– Engine noises
– Oil pressure problems
– Cooling system problems
• Causes of engine problems– Normal wear
– Lack of maintenance
– Previous work
– Problems in other areas
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Diagnosing Problems Before a Repair
• Diagnose engine before disassembly– Determine repair is necessary
– Determine exact location while engine running
• Discuss problem with vehicle’s owner– Driving habits or lack of maintenance may be the
cause
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Oil Consumption• Oil lost is through external leakage or internal oil
consumption– Internal consumption: spotted by oily coating on
inside of exhaust pipe or blue smoke• Overly rich air-fuel mixture causes black soot on
exhaust pipe and black smoke
• Normal oil consumption – Depends on size of engine, vehicle weight,
shape, etc.
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Causes of Oil Consumption• Bad valve guides or seals
– Smoke visible from exhaust during deceleration
• Worn compression rings– Frequent cause: poor maintenance
• Increased consumption after a valve job– Consider entire engine
• Excessive rod bearing clearance– Engines with high mileage
• Vacuum modulator– Older automatic transmissions
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Causes of Oil Consumption (cont'd.)
• Incorrect oil level– Incorrect dipstick size causes overfilling
• Plugged cylinder head drainback holes– Poor maintenance
• Leaking V-type intake manifold gasket– Difficult problem to find
• Crankcase pressure – Plugged PCV valve
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Testing for Oil Leaks • Oil can leak past gaskets and seals
– Rear main bearing seal leak• Oil on engine side of flywheel or torque converter
– Front transmission seal leak• Oil on transmission side of torque converter
• Black light testing– Add one ounce of florescent liquid to oil
– Drive the car
– Use a black light and a mirror to find leaks
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Engine Performance and Compression Loss
• Compression loss causes– Blown head gasket
– Burned valves
– Broken piston rings
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Engine Noises• Determine noise location before disassembly
– Noises can be transmitted from their origins to other locations
• Difficult to isolate
• Accessories can cause noises– Inspect alternators, smog pumps, air-
conditioning compressors, and coolant pumps
– Belts a common source of noise
– Fan clutch on coolant pump can sound serious• Difficult to locate
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Engine Knocks• Crankshaft noises: generally deeper in pitch
– Front main bearing knock
– Thrust bearing knock
– Rod knock
– Related noises (e.g., loose flywheel, torque converter, and vibration damper)
– Bent oil pan
– Rod side clearance
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Engine Knocks (cont’d.)• Piston noises
– Cracked pistons
– Piston slap
– Excess piston pin clearance
– Other piston sounds
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Engine Knocks (cont’d.)• Valve train noises: loud ticking sound
– Sticking valve
– Worn or flat cam lobe
– Timing components
• Lifter noises: occur when engine is first started– Intermittent noise at idle or low speed
– Noise at idle that goes away at higher speeds
– Quiet at idle but noisy at high speed
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Engine Knocks (cont’d.)• Lifter noise at all engine speeds
– Dirt or varnish buildup
– Worn parts or insufficient oil supply
– Oil is too thin or pressure is too low
• Spark knock noise– Several causes
– Excessive carbon buildup
• Broken motor mount– Check for engine lift when transmission is in
forward and reverse ranges with brakes applied
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Oil Pressure Problems• Low oil causes major engine damage
– Lower main bearing wear: oil pressure permanently low at idle
• Low oil pressure– Faulty oil pressure sending unit
• High oil pressure– Stuck pressure relief valve
– Severe blockage in oil gallery
• Oil analyzed in a lab – Identifies mechanical problems
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Cooling System Problems• Neglected cooling system
– Results in expensive engine damage
• Plugged or corroded radiator– Cannot conduct heat away from engine
– Overheats at freeway speeds
• Water jackets develop buildup of minerals and scale– Prevents heat transfer
– Material flakes off and plugs radiator
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Internal Engine Leakage• Locations of internal leaks
– Water crossover passage of intake manifold
– Threaded plugs beneath valve covers
– Combustion chamber
– Cracked cylinder block
• Diagnosed using:– Block tester, pressure tester, or infrared analyzer
• Cross fluid contamination – Water leaking into crankcase contaminates oil
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Internal Engine Leakage (cont'd.)
• Internal oil to coolant leaks– Leak between oil and water passageway causes
pressurized oil to leak into cooling system• Spotted by installing pressure tester on radiator
filler neck
– Leaking head gasket may not show up on a pressure test
• Block check tester or infrared exhaust analyzer checks for exhaust gas in coolant
• Bubbles in coolant indicate a leak
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Seized Engine• Starter motor will not crank the engine
– Engine cannot be cranked by hand
• Frozen accessory can prevent engine from cranking– Drive belt can become so hot it melts
• Coolant thermoplastic seizure– Coolant mixes with engine oil
• Hydrolock – Both cylinder valves are closed
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Electronic Failures/Engine Damage
• Engine damage may be traced to electronic component failures– EGR valve becomes inoperative if its input
sensor signals interrupted
– Electric cooling fan failure can be due to inoperative sensor
– Overly rich air-fuel mixture can cause oil dilution
• Always trace a problem to its root cause
© 2012 Delmar, Cengage Learning
Engine Performanceand Fuel Mixture Problems
• Emission control and fuel system malfunctions– Mimic problems related to the engine
• Lean air-fuel mixture – Increases heat in combustion chamber
• Results: detonation or burned internal engine parts
• Rich air-fuel mixture – Causes oil wash
• Oil washed from cylinder walls• Leaking fuel injectors also cause oil wash