Diagnosing Engine Mechanical Problems

1. Diagnosing Engine Mechanical Problems Chapter 49

2. Objectives • Use engine diagnostic tools and equipment safely and properly • Diagnose engine and related problems prior to repair

3. 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

4. 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

5. 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.

6. 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

8. 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

9. 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

11. Engine Performance and Compression Loss • Compression loss causes • Blown head gasket • Burned valves • Broken piston rings

12. 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

13. 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

14. Engine Knocks (cont’d.) • Piston noises • Cracked pistons • Piston slap • Excess piston pin clearance • Other piston sounds

15. 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

16. 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

17. 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

19. 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

20. 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

21. 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

23. 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

24. 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

25. 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