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Engine Problem Diagnosis and Tune-Up
Chapter 19
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Objectives
Describe a minor tune-up and a major tune-up.
List tune-up safety rules.
List items to look for in a preliminary inspection.
Perform a current starter draw test.
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Objectives
Perform dry and wet compression tests and analyze the results.
Perform a cylinder leakage test and analyze the results.
Perform a vacuum test and analyze the results.
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Objectives
Perform a cylinder balance test and analyze the results.
Describe valve adjustment.
List general tune-up rules.
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What Is a Tune-Up?
Engine tune-up returns engine to peak performance after part wear and deterioration
Tune-up involvesReplacing worn parts
Performing basic service tasks
Making adjustments
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Minor Tune-Up
Minor tune-up is preventative maintenancePerformed when engine is in good condition
Engine has little or no wear
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Minor Tune-Up (Cont.)
Visually inspect for problems
Listen for abnormal noises
Check condition of engine oil and coolant
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Minor Tune-Up (Cont.)
Check for trouble codes in computer
Replace distributor cap, rotor, and spark plugs (older, high-mileage vehicles)
Check and adjust ignition timing
Replace air and fuel filters
Make fuel system adjustments where applicable
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Minor Tune-Up (Cont.)
Clean throttle body, idle air bypass, and EGR system
Test and service emission control system
Fill all fluid reservoirs to correct level
Road test the vehicle
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Major Tune-Up
Major tune-up is done when systems or engine is in poor condition and includes
All parts of minor tune-up
Diagnostic tests to determine what should be done to bring vehicle back to good operating condition
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Tune-Up Safety
Engage emergency brake and block wheels when engine is to be running
Place exhaust hose over tailpipe when running engine in enclosed shop
Keep clothing, hands, tools, and equipment away from moving parts
Disconnect battery when recommended
Be careful not to touch hot exhaust manifold
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Tune-Up Safety (Cont.)
Keep test equipment leads away from exhaust manifolds, fan, and beltsWear eye protection when blowing debris from parts or when working near engine fanKeep fire extinguisher handy, especially during fuel system tests and repairsWith diesel engine, disable injection pump when removing injection line
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Preliminary Inspection
Look for problems withBattery
Air cleaner
Fuel system
Belts and hoses
Poor electrical connections
Fix problems before proceeding with tune-up
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Evaluating the Engine and Its Systems
If scan tool indicates problem with oxygen sensor, remove and inspect sensor
Note color of tip of oxygen sensorColor can indicate condition of engine and support systems
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Evaluating the Engine and Its Systems (Cont.)
(Tomco, Inc.)
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Evaluating the Engine and Its Systems (Cont.)
If engine is running too hot or cold, measure operating temperature
Use engine analyzer or handheld tester to check for blown head gasket
To find hard-to-locate oil and coolant leaks, add dye and use a black light
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Cranking Balance Test
Cranking balance test finds cylinders with low compression
Monitors current drawn by starter motor during complete turn of crankshaft
Low current draw indicates that starter is not being loaded enough by compression stroke
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Compression Test
Compression test determines engine’s mechanical conditionLow compression may be caused by
Burned valve or valve seatPhysical engine damageBlown head gasketWorn rings or cylindersValve train troublesJumped timing chain or belt
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Compression Test—Gasoline Engine
Remove all spark plugsBlock open throttle platesDisable ignition systemDisable fuel injection systemScrew compression gauge into spark plug holeCrank engine and let it rotate for about six compression strokes
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Compression Test—Gasoline Engine (Cont.)
Write down readings for each cylinder
Compare readings to specifications
(Snap-on Tool Corp.)
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Compression Test—Diesel Engine
Procedure is similar to compression test for gasoline engine
Do not use compression gauge intended for gasoline engine on diesel engine
Diesel compression gauge must read up to 600 psi (4200 kPa)
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Compression Test Results
Gasoline engine readings should be 125-175 psi (860-1200 kPa) in each cylinder
Diesel engine readings should be 275-400 psi (1900-2750 kPa)
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Compression Test Results (Cont.)
Look for cylinder pressure variations during compression check
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Wet Compression Test
Wet compression test should be done on any cylinder with compression reading below specifications
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Wet Compression Test (Cont.)
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Cylinder Leakage Test
Cylinder leakage test measures air leakage out of engine combustion chamber
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Vacuum Test
Vacuum test measures intake manifold vacuum
(General Motors)
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Cylinder Balance Test
Cylinder balance test disables spark plug in one cylinder at a time
Tests whether cylinders are firing properly
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Valve Adjustment
On some engines, valve adjustment involves turning rocker arm adjusting screws
(Volvo)
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Valve Adjustment (Cont.)
On some OHC engines, valve adjustment involves changing shim thickness to alter valve lash
(Ford)
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Tune-Up Parts Replacement
Depending on age and condition of engine, parts may need replacement during tune-up
On newer vehicles, only spark plugs and filters may need replacement
Older vehicles with high mileage may need multiple parts
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Tune-Up Parts Replacement (Cont.)
Parts that may need replacing includeSpark plugs
Spark plug wires
Distributor parts
Fuel system parts
Emission control parts
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Tune-Up Adjustments
Adjustments needed will vary with make, model, and condition of vehicle
Most common adjustments includeSpark plug gap
Pickup coil air gap
Ignition timing or injection timing
Idle speed
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Tune-Up Adjustments (Cont.)
(Snap-on Tool Corp.)
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General Tune-Up Rules
Gather information about performance of engine
Use professional, high-quality tools and equipment
Make sure engine has warmed to full operating temperature
Refer to service information or emission sticker for specifications and procedures
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General Tune-Up Rules (Cont.)
Use quality parts
Keep service records
Complete basic maintenance service
Inform customer about tune-up intervals
Road test the vehicle when tune-up is complete
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Diesel Engine Tune-Up(Maintenance)
Diesel maintenance involvesReplacing air filter element
Cleaning, draining, or replacing fuel filters
Adjusting engine idle speed and throttle cable
Inspecting engine and related systems
Checking injection timing
Changing engine oil and oil filter
Servicing emission control systems periodically
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Review Questions
A major tune-up consists of a minor tune-up plus which of the following?
A. Oil change
B. Coolant change
C. Battery recharge
D. Diagnostic tests
Answer: D. Diagnostic tests
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Review Questions
When working on a diesel engine, disable the ____.
Answer: injection pump
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Review Questions
True or False? Checking for fuel system leaks is part of the preliminary inspection.
Answer: True
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Review Questions
A cranking balance test will locate cylinders with ____ compression.
Answer: low
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Review Questions
True or False? A wet compression test is used to locate defective valves.
Answer: False. A wet compression test is used to locate defective rings.
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Review Questions
In a(n) ____ test, shop air is pumped into a cylinder and a pressure gauge is used to measure air leakage.
Answer: cylinder leakage
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Review Questions
What would cause a vacuum gauge to show higher than normal readings while idling in neutral?
A. Burned exhaust valveB. Burned intake valveC. Worn oil ringsD. None of the above.Answer: D. None of the above.
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Review Questions
What test is commonly performed to find out if individual cylinders are firing properly?
Answer: Cylinder balance test
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Review Questions
In some ____ engines, valves must be adjusted by changing the shim thickness.
Answer: OHC
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Review Questions
True or False? Diesel engines should be tuned up at regular intervals, just like gasoline engines.
Answer: False. Diesel engines do not require periodic tune-ups.