chapter 6 diagnosis

7/30/2019 Chapter 6 Diagnosis

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Engine diagnosis

On-board diagnostics

Codes for misfires (OBDII)

Functional tests for power balance (Prior to OBDII)

Further considerations

Mileage

Oil consumption

Noises

Copyright 2003 Gary Lewis - Dave Capitolo


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Check engine performance

Valve-to-seat leakage causes . . .

Rough idle

Power loss

Timing chain/belt failure can cause . . . No start

Rough idle

Mistaken for valve problem

Misfiring cylinder causes . . . Rough idle

Power loss


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Check engine performance

Engine oil problems

Leaks

Oil consumption

Misfiring from fouled spark plugs Smoke on accelerationworn rings

Smoke on decelerationworn guides

Valve related noises

Popping at exhaust pipe - leaking exhaust valve Backfire through intakeleaking intake valve or

exhaust valve not opening

Valve lash noisesadjustment or wear in valve train


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Effects of temperature, frequency, and oil pressure

Temperature

Piston noises are effected by temp

Piston noises are reduced with heat

Frequency Noises at crankshaft speed are valve train

Noises at crankshaft speed are piston/rod assm.


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Diagnostic techniques

Block check

Draw vapor from radiator through solution

Turns yellow if combustion gases are present

Exhaust gas analyzer

Place probe OVER radiator filler neck

Check for HC

Oil pressure Noises at crankshaft speed that diminish with

oil pressure are due to rod bearing clearance

Noises at crankshaft speed that diminish with

oil pressure are from hydraulic lifters


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Power balance test

To find cylinders with poor contribution

Raise engine RPM and ground plug wire

RPM drop of suspect cylinder will be less than the rest

Power balance test using Scan tool


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Compression test

To determine if rings or valves are faulty

Results Record first and last readings

Add oil to low cylinders & retest

High results are from carbon build up

Low readings on adjacent cylindershead gasket

Set up

Warm engine Remove plugs

Disable ignition or fuel

Connect tester

Specs

10% variation 20 PSI difference

75% of high cyl


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Cylinder leakage test

To pinpoint rings, intake, or exhaust valves are faulty

Results 20% to 30% considered poor

Normal leakage increases with bore diameter

Set up

Piston at TDC compression Add shop air (100PSI)

Open throttle

Remove oil filler cap

Air leaks . . .

Intake systemintake valve

Tail pipeexhaust valve

Crankcaserings

Radiatorcrack or gasket


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Vacuum testing

Results

Low steady readingsvacuum leak, late ignitiontiming, or worn rings

Intermittently droppingsticking valves

Rapid needle movementleaking valves

Rapid needle movement on accelvalve springs

Set up

Connect gauge to manifold vacuum

Shortest possible hose Normal between 17 and 21 Hg. Minus 1 per

1000 ft. elevation increase


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Vacuum testing

Ring leakage & exhaust restriction

Note vacuum at idle

Hold engine at 2,000 RPM Release throttle rapidly & watch vacuum rebound

< 3 inches of rebound indicates worn rings

Slowly returning needle indicates exhaust restriction

Check vacuum at 3,000 RPM to confirm restriction


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To check timing chain backlash

Rotate engine forward until timing mark is at TDC

Mark the position of ignition rotor

Rotate engine opposite direction until rotor moves Read back lash amount on timing marks


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To check valve timing

Timing marks on sprockets

#1 at TDC compression

Companion at TDC exhaust

Exhaust backpressure

Restricted exhaust increases backpressure

With no engine load should not exceed 1.5 PSI


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Engine noises

Valve train noises Use stethoscope at each rocker

Anything that runs at camshaft speed

Excessive main bearing clearance

Deep metallic knock Low oil pressure

Most noticeable under heavy load

Excessive crankshaft endplay Sharp metallic rap

More noticeable when pressing or releasing clutch


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Engine noises

Excessive connecting rod clearance Metallic rap

Noticeable under light load

Frequency increases with engine RPM

Isolate by grounding plug wireExcessive piston clearance

Piston slap is heard as dull rattle

May diminish with temperature increase

Excessive piston pin clearance

Light metallic rap at idle and low speeds

Isolate by grounding plug wirefrequency


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Testing oil pressure

Oil pressure facts Engines can idle with < the oil pressure at cruise

Oil pressure can drop in once the engine warms up

Some engines have less than 15 PSI at warm idle

chapter 6 diagnosis

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