Engine Performance and Driveability

by
Russell Krick

Publisher

The Goodheart-Willcox Co., Inc.
Tinley Park, Illinois
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Permission granted to reproduce for educational use only


© Goodheart-Willcox Co., Inc.

Permission granted to reproduce for educational use only


 Locating engine performance problems
 Typical performance problems

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Engine performance problems, often
called driveability problems, affect power,
fuel economy, emission output
levels, and dependability

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Strategy-Based
Diagnostics
 Use a systematic approach
 Question the driver or service advisor
about the symptoms and when they
occur
 Look for the root cause of the failure

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StrategyBased
Diagnostics

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Diagnostic Example
 An injected engine misses and emits
heavy black smoke after cold starting
 Heavy black smoke = too much fuel
 Only when cold = temperature related

 Engine coolant temperature sensor?
 ECM increases fuel when cold

 Temperature-related input (ECT sensor)
may be the root cause
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Common Trouble
Sources

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Troubleshooting
Charts
 If you have trouble locating an engine
performance problem, refer to a service
manual troubleshooting chart
 Troubleshooting charts are based on
symptoms
 Useful when no diagnostic trouble
codes are present
 Suggests possible causes based on
symptoms
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Troubleshooting Chart
Fuel Supply System

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Technical Service
Bulletins
 TSBs
 Explain problems that frequently occur
in one make and model of vehicle
 Published by manufacturers
 Explain symptoms, tests, and
corrections for common problems

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© Goodheart-Willcox Co., Inc.

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No-Crank
 The starter fails to turn the crankshaft
 Starting system or engine mechanical
problem
 starter, solenoid, and related wiring

 The starter spins but the engine does
not turn
 starter drive or ring gear teeth

 Mechanical problem suspected
 turn the engine over manually to verify
that it turns
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No-Start
 The engine cranks but fails to start
 Check for spark while cranking
 use a spark tester

 Check for fuel while cranking
 TBI—watch for injection
 MFI—check fuel pressure and use a noid
light to check the injector wiring

 Check compression

 rings, valves, and valve timing are factors
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Check for Spark

A bright spark should jump the gap
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Check for Fuel

Noid light flashes, verifying ECM
control—check the fuel pressure
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Hard Starting
 Starting may be affected by several
systems
 starting motor not turning quickly enough
 vacuum leaks
 engine mechanical problems
 incorrect fuel mixture due to a fuel

system problem
 poor spark or incorrect timing due to an
ignition system problem
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Stalling
 Engine stops running at idle, after cold
starting, or after warm-up
 Causes:
 low idle speed
 injection system or carburetor problem
 ignition system trouble
 severe vacuum leaks

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Misfiring
 One or more cylinders failing to fire
normally
 at idle, under acceleration, or at cruise

 Causes of misfire at idle:
 fouled spark plugs or open plug wires
 a cracked distributor cap or corroded

terminals
 vacuum leaks
 a problem with the carburetor’s idle circuit
or its adjustment
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OBD II Misfire Monitor
 Misfires can damage the catalytic
converter
 When a cylinder misses, crankshaft
acceleration will slow down slightly with
the misfire
 The ECM monitors the crankshaft
position sensor for this condition

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Scan Tool Misfire Data
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Misfire data values
Misfire history
Misfire passes
Misfire failures
Misfire rev. status

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Total misfires
Misfiring cylinder
RPM at misfire
Load at misfire

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Rough Idle
 Engine seems to vibrate
 Popping noise at the tailpipe caused by
some of the cylinders not firing
normally
 Causes:
 vacuum leaks
 lean air-fuel mixture
 spark plugs and wires

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Engine Performance
Problems

Engine performance over time
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Hesitation
 The engine does not accelerate
normally
 Flat spot before developing power
 Often a temporarily lean condition
 Causes:
 faulty acceleration-related components
 faulty throttle position sensor
 faulty accelerator pump on carbureted
systems
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Stumbling
 Engine misses, causing jerking on
acceleration
 Causes:
 faulty oxygen sensor
 faulty throttle position sensor
 faulty engine coolant temperature sensor
 intermittent electrical problems causing
loss of power to systems
© Goodheart-Willcox Co., Inc.

Permission granted to reproduce for educational use only