Engine Performance and Driveability

Permission granted to reproduce for educational use onlyEngine performance problems, often called driveability problems, affect power, fuel economy, emission output levels, and dependabi

The Goodheart-Willcox Co., Inc.

by

Russell Krick

© Goodheart-Willcox Co., Inc Permission granted to reproduce for educational use only

 Locating engine performance problems

 Typical performance problems

© Goodheart-Willcox Co., Inc Permission granted to reproduce for educational use only

Engine performance problems, often called driveability problems, affect power,

fuel economy, emission output

levels, and dependability

Strategy-Based

Diagnostics

about the symptoms and when they occur

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Strategy-Based Diagnostics

Diagnostic Example

heavy black smoke after cold starting

 ECM increases fuel when cold

may be the root cause

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Common Trouble

Sources

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

Technical Service

Bulletins

 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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 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

Check for Spark

A bright spark should jump the gap

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Check for Fuel

Noid light flashes, verifying ECMcontrol—check the fuel pressure

 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

normally

 at idle, under acceleration, or at cruise

 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

Scan Tool Misfire Data

 Misfire data values

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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

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

 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

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Surging

 Engine power fluctuates up and down

with a continuous, soft jerking motion while driving at a steady speed

 Causes:

 extremely lean fuel injection or carburetor

 problems with the ignition or computer control systems

 problems with an electronically controlled automatic transmission

 intermittent electrical problems

 Air-fuel mixture igniting in the intake

manifold or exhaust system

 Loud bang or pop sound

 Causes:

 incorrect ignition timing, crossed spark plug wires, or a cracked distributor cap

 faulty ignition coil

 faulty throttle position sensor or computer

 faulty air injection system

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Dieseling or Run-On

 The engine fails to shut off, keeps firing

after the ignition has been turned off

 Causes:

 high idle speed, overheated engine, low octane fuel, or cylinder carbon buildup

Pinging (Spark Knock)

 Metallic tapping or light knocking sound

on acceleration under load

 Abnormal combustion (preignition,

detonation, or spark knock)

 Causes:

 low octane fuel

 advanced ignition timing

 carbon buildup

 overheating

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Vapor Lock

 Fuel is overheated, forming bubbles

that upset mixture control

 Causes stalling, hard starting, or loss of

Fuel Line Freeze

 Moisture in the fuel turns to ice

 Blocks the filters and sharp bends in

the fuel system

 Prevents engine operation

 Diesel fuel can form wax in cold

weather

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Poor Fuel Economy

 Vehicle uses too much fuel for the

Lack of Power

 Sluggish engine, causes vehicle to

accelerate slowly

 Causes:

 fuel system problems

 ignition system problems

 emission control problems

 engine mechanical problems