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
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
Check engine performance
Engine oil problems
• Leaks
• Oil consumption
• Misfiring from fouled spark plugs
• Smoke on acceleration – worn rings
• Smoke on deceleration – worn guides
Valve related noises
• Popping at exhaust pipe - leaking exhaust valve
• Backfire through intake – leaking intake valve or
exhaust valve not opening
• Valve lash noises – adjustment or wear in valve train
Effects of temperature, frequency, and oil pressure
Temperature
• Piston noises are effected by temp
• Piston noises are reduced with heat
Frequency
• Noises at ½ crankshaft speed are valve train
• Noises at crankshaft speed are piston/rod assm.
Diagnostic techniques
Oil pressure
• Noises at crankshaft speed that diminish with
oil pressure are due to rod bearing clearance
• Noises at ½ crankshaft speed that diminish with
oil pressure are from hydraulic lifters
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
Diagnostic techniques
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
Diagnostic techniques
Compression test
• To determine if rings or valves are faulty
Set up
Specs
• Warm engine
• 10% variation
• Remove plugs
• 20 PSI difference
• Disable ignition or fuel
• 75% of high cyl
• Connect tester
Results
• Record first and last readings
• Add oil to low cylinders & retest
• High results are from carbon build up
• Low readings on adjacent cylinders – head gasket
Diagnostic techniques
Cylinder leakage test
• To pinpoint rings, intake, or exhaust valves are faulty
Set up
• Piston at TDC compression Air leaks . . .
• Add shop air (100PSI) • Intake system – intake valve
• Tail pipe – exhaust valve
• Open throttle
• Crankcase – rings
• Remove oil filler cap
• Radiator – crack or gasket
Results
• 20% to 30% considered poor
• Normal leakage increases with bore diameter
Diagnostic techniques
Vacuum testing
Set up
• Connect gauge to manifold vacuum
• Shortest possible hose
• Normal between 17” and 21” Hg. Minus 1” per
1000 ft. elevation increase
Results
• Low steady readings – vacuum leak, late ignition
timing, or worn rings
• Intermittently dropping – sticking valves
• Rapid needle movement – leaking valves
• Rapid needle movement on accel – valve springs
Diagnostic techniques
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
Diagnostic techniques
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
Diagnostic techniques
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
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
Engine noises
Excessive connecting rod clearance
• Metallic rap
• Noticeable under light load
• Frequency increases with engine RPM
• Isolate by grounding plug wire
Excessive 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 wire – frequency 
Testing oil pressure
Oil pressure facts
• Engines can idle with < ½ the oil pressure at cruise
• Oil pressure can drop in ½ once the engine warms up
• Some engines have less than 15 PSI at warm idle