chapter 48 engine problems
TRANSCRIPT
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Publisher
The Goodheart-Willcox Co., Inc.Tinley Park, Illinois
by
Russell Krick
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Why is diagnosis important? Symptoms of engine mechanical
problems Decide what type of engine repair is
needed Evaluating engine mechanical
problems Service manual troubleshooting charts
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If a technician does not know how to properly diagnose engine problems, time, effort, and money will be wasted
An untrained technician may rebuild an engine when a minor repair would have corrected the fault
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Excessive oil consumption Excessive crankcase blowby Noises Exhaust smoke Poor performance Coolant in the oil Engine seizure
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Engine Mechanical Problems
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Pre-Teardown Inspection
Run the engine Look for external problems:
oil leaks vacuum leaks part damage contaminated oil
Listen and watch for problems
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Check Oil Condition Check for coolant in the oil
shows up as white or milky oil
Check for gasoline in the oil smell the dipstick for gasoline
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Oil-Fouled Spark Plugs
Indicate internal oil leakage into the combustion chambers—caused by worn
rings, cylinder walls, or valve seals
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Oil In Coolant
Causes: a leak in the radiator oil cooler head gasket leakage a cracked block or head
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Engine Oil Leaks Causes:
gaskets harden and crack seals wear fasteners work loose parts become warped or cracked
To isolate leaks: clean the affected area trace the leak upward to its source
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External Coolant Leaks
Causes: hose problems rusted freeze (core) plugs warped, worn, or damaged parts
Use a pressure tester to locate leaks
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Engine Blowby
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Engine Vacuum Leaks
May produce a hissing sound Affect idle quality more than high speed
conditions leaks represent a greater percentage of
engine airflow at idle than at high speed
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Engine Exhaust Leaks
Produce a clicking sound Causes:
leaking exhaust gaskets a warped exhaust manifold loose manifold bolts
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Engine Exhaust Leaks
The leakage path can be easily seen after exhaust manifold removal
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Engine Smoking
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Abnormal Engine Noises
Indicate part wear or damage Use a stethoscope to find internal
noises A piece of hose can be used
place one end next to your ear, use the other end to probe around the engine
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Compression Test Measures the amount of pressure
produced during the compression stroke
Performed any time symptoms point to cylinder pressure leakage rough idle popping noise in intake or exhaust blue exhaust smoke excess blowby
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Combustion Leakage
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Compression Testing(Gasoline Engine)
Remove all spark plugs Block open the throttle Disable the ignition and fuel injection Screw the compression tester into a
spark plug hole Crank the engine through 4–6
compression strokes, noting the readings
Repeat for each cylinder
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Compression Tester
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Installing Adapter
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Connect Compression Gauge
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Cranking Engine
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Replacing Plugs and Wires
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Compression Testing(Diesel Engine)
Use a diesel compression gauge that reads up to approximately 600 psi(4000 kPa)
Remove the injectors or the glow plugs Install the tester in the recommended
hole Disable the injection pump Crank the engine, noting the readings Repeat for each cylinder
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Wet Compression Test Performed if a cylinder fails a
compression test Helps isolate cylinder and ring
problems from valve problems Oil is squirted into the cylinder before
the compression test if the pressure rises, the rings and
cylinder are suspect if the pressure does not rise, the valves
are suspect
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Wet Compression Test
A. Perform dry test B. Squirt oil into cylinder
C. Measure again
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Compression Test Results
Refer to service manual for specifications
Typical gasoline engines 125–175 psi (860–1200 kPa)
Typical diesel engines 275–400 psi (1900–2750 kPa)
Maximum variation 10–15% between cylinders
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Cylinder Leakage Tester
Measures the amount of air leakage out of a cylinder
Shop air pressure is forced into the cylinder on Top Dead Center
Pressure gauge reads percentage of leakage out of the cylinder
Typical maximum leakage is 20%
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Cylinder Leakage Tester
If leakage exceeds specifications, listen at various points to isolate leakage
Oil filler cap—ring leakage Throttle body—intake valve leakage Exhaust pipe—exhaust valve leakage Radiator bubbles—head gasket or
cracked head, block
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After performing inspections and tests, decide what part or parts must be repaired or replaced
Evaluate your pre-teardown diagnosis If you still can’t determine the problem,
partially disassemble the engine for further inspection
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Valve Train Problems
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Burned Valve
Causes an engine miss, a popping sound at the throttle body or exhaust
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Worn Valve Guidesand Stems
Causes tapping noise, oil consumption, spark plug fouling, or stem
breakage
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Leaking Valve Stem Seal
Lets oil drain through the clearance between the stem and guide
Oil will be pulled into the intake port and burned
Causes blue exhaust smoke, especially after startup
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Valve Breakage
Caused by valve stem fatigue or by a broken or weak valve spring
Usually causes severe piston and cylinder damage
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Stuck Valve
Valve stem rusts or corrodes and locks in the valve guide
May happen when the engine sits in storage
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Valve Float
Excess engine speed, weakened valve springs, or lifter problems cause the valves to remain partially open
Usually occurs at higher engine speeds Engine may miss, pop, or backfire
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Worn Timing Chain
Causes reduced compression and power
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Worn Timing Belt
May break, jump off its sprockets, or skip a few teeth
Severe lack of power, no-start, and valve damage can result
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Camshaft Problems
Worn cam lobes Worn journals Broken cams Worn distributor drive gear Loose or worn fuel pump drive
eccentric
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Cam Lobe Wear
Reduces valve lift and power output
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Rocker Arm and Push Rod Problems
May cause clatter (light tapping noise) Rocker arms may wear Push rods may be bent To check, roll the push rods on a flat
bench
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Valve Train Clearance
Excess clearance can produce noise
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Hydraulic Lifter Problems
Worn or defective hydraulic lifters may produce valve clatter
To check, remove the valve cover Try adjusting the valves If adjustment will not quiet the noise,
check for valve train wear If the valve train passes, the lifter is bad Low oil pressure can cause lifter noise
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Piston Knock (Slap)
Loud, metallic knock that is loudest whenthe engine is cold
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Piston Pin Knock
Too much clearance between the piston pin and the pin bore or connecting rod bushing
Makes a double knock The noise does not change much with
engine load
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Worn Rings and Cylinder
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Worn Ring Grooves
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Burned Piston
Results from prolonged preignition or detonation
Causes low compression, blowby, smoking, and rough idle
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Burned Piston
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Crankshaft Problems
Journal wear Main bearing wear Rod bearing wear Low oil pressure
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Rod Bearing Knock Light, regular rapping noise with the
engine floating Loudest after warm-up Caused by wear and excessive rod
bearing-to-crankshaft clearance To locate, short out or disconnect the
spark plug wires, one at a time The knocking bearing may quiet down
when its cylinder is disabled
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Main Bearing Knock
Similar to rod bearing knock, but slightly deeper in pitch
More pronounced when the engine is under load
Worn bearings and journals are letting the crankshaft move up and down
Usually reduces oil pressure
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Main Bearing Knock
To verify, remove the oil pan and pressure test the lubrication system
Excessive oil flowing out of one or more of the main bearings implies too much bearing clearance
If the crankshaft is not worn, the bearing inserts may be replaced
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Excess Crankshaft End Play
Caused by a worn main thrust bearing May produce a deep knock when
applying or releasing the clutch On an automatic transmission, a single
thud or knock may occur during acceleration or deceleration
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Broken Engine Mounts Allow the engine to move in the vehicle To check:
open the hood engage the parking brake shift transmission into drive or into gear with the brakes on, slowly increase
engine speed or release the clutch pedal if the engine moves excessively, the
mounts may be broken
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Broken Engine Mounts
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Charts list possible problems and needed repairs
Refer to the chart when you have difficulty locating or correcting an engine mechanical problem