Diagnosing Engine Performance Problems

1
Diagnosing Engine Performance Problems

• Chapter 48

2
Objectives

• Perform compression tests in a correct manner
• Perform cylinder leakage tests in a correct
  manner
• Describe the procedures for doing various vacuum
  tests
• Explain the differences in types of carbon
  deposits

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Introduction

• Engine and ignition system problem results
• Poor fuel economy
• High emissions
• Poor driveability
• Requirements for engine to run and start properly
• Sufficient compression
• Timed spark
• Fuel and air mixed in the correct ratio

4
Visual Checks

• Perform visual check when an engine does not
  start or is running poorly
• Vacuum line
• Electrical wires
• Fuel gauge
• Believe it or not, sometimes the vehicle is out
  of gas

5
Ignition System Checks

• Ignition system must produce a strong spark at
  the correct moment
• Checking for spark is quickest thing to do
• Fuel injection
• Easier to test compression than check for fuel on
  port injection engines
• Sometimes engine starts and runs but one cylinder
  has a dead misfire
• Ignition or mechanical problems

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Engine Performance Testing

• Fuel problems
• Dry black soot at exhaust pipe
• Fuel in the crankcase
• Air-fuel mixture
• Can be affected by air leaks
• Other problems and causes
• Poor quality fuel blowby and EGR gases cause
  gummy deposit behind throttle plate
• Dirty fuel injectors cause driveability problems
• Plugged fuel filter causes starvation at high rpm

8
Compression Loss

• Causes of low compression
• Engine breathing problems
• Compression leaks
• Breathing problem causes
• Carbon buildup around neck of valve
• Intake restrictions
• Blocked exhaust
• Catalytic converters operating for a long period
  when there is a misfire
• Become plugged

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Vacuum Testing

• Intake manifold vacuum readings
• Useful in determining engine problems
• Leaking intake manifold
• Causes rough idle
• Low, steady vacuum
• Check ignition timing
• Leaking valve
• Indicated when needle drops at regular intervals

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Vacuum Testing (cont'd.)

• Restricted exhaust systems
• Cause hissing sound
• Cranking vacuum test
• Performed to check for internal leaks or
  incorrect valve timing

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Other Vacuum Tests

• V-type engines
• Can have a vacuum leak
• Test pinch off PCV valve hose to the manifold
  and breather hose to the air cleaner
• Vacuum at oil filler opening internal vacuum
  leak
• Vacuum signals
• Can be used with a digital storage oscilloscope
• Smoke tester
• Finds vacuum, exhaust, cooling, and oil leaks

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Compression Problems

• Compression leak causes
• Blown head gasket
• Burned valves
• Worn or broken piston rings
• Fast way of locating a weak cylinder
• Cylinder power balance test
• Compare performance of cylinders
• Compression test
• Pinpoints compression problems

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Compression Problems (cont'd.)

• Causes of variations in cylinder
• Ignition or fuel system
• Engine vacuum leaks
• Compression problems
• Hand-held scan tools
• Have power balance test capability
• Compression tester
• Common piece of equipment

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Compression Problems (cont'd.)

• Compression test procedure
• Twist rubber boots on spark plug cables
• Before removing spark plugs, blow dirt away from
  their base
• Block the throttle in wide-open position
• Connect remote starter switch
• Insert compression gauge into spark plug hole and
  crank engine
• When cranking, disable ignition system

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Compression Problems (cont'd.)

• Interpreting compression test results
• Check manufacturers tolerance specifications
• Two cylinders next to each other have low
  compression indicates blown head gasket
• Burned exhaust valves can cause of one or more
  cylinders with low compression
• All cylinders have low compression incorrect
  valve timing could be the cause
• Cylinders with compression higher than specified
  can result in carbon buildup

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Compression Problems (cont'd.)

• Wet compression test
• Used if all cylinders show poor results
• Oil is squirted on each cylinder
• Running compression test
• Measures how well the cylinders can draw in air
  and fuel
• Running compression should be about 80 of
  cranking compression

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Cylinder Leakage Test

• Used to pinpoint causes of leakage in a
  combustion chamber
• Leaks can be pinpointed by listening in different
  places
• Oil filter leaking rings or pistons
• Air cleaner leaking intake valve
• Exhaust leaking exhaust valve
• Bubbles in radiator blown head gasket or crack
  in head or block

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Cylinder Leakage Test (cont'd.)

• Advantages of leakage test
• Performed on an engine removed from a vehicle
• Performance of camshaft will not affect the
  results
• Source of leakage pinpointed before engine
  disassembly

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Carbon-Related Problems

• Carbon causes performance problems
• Increased compression ratio
• Fuel absorption
• Blocked air and fuel flow
• Kinds of deposits
• Oil based and carbonaceous
• Carbonaceous deposits
• Result from fuel
• Are difficult to remove and cause driveability
  problems