Diagnosing Engine Mechanical Problems

© 2012 Delmar, Cengage Learning

Diagnosing Engine Mechanical Problems

Chapter 49


Objectives• Use engine diagnostic tools and equipment

safely and properly• Diagnose engine and related problems prior to

repair


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


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


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.


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



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


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



Engine Performance and Compression Loss

• Compression loss causes– Blown head gasket– Burned valves– Broken piston rings


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


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


Engine Knocks (cont’d.)• Piston noises

– Cracked pistons– Piston slap– Excess piston pin clearance– Other piston sounds


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


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


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



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


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


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



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


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


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

Documents

Diagnosing Engine Mechanical Problems