Problem & Possible Cause

Action

Carbon Fouled Plugs

Clogged Air Filter

Replace Air Filter

Incorrect Idle Speed

Reset Idle Speed

Faulty Ignition Wiring

Replace Ignition Wiring

Sticky Valves/Worn Valve Seal

Check Valve Train

Fuel Injection Operation

Check Fuel Injection

Wet/Oil Fouled Plugs

Worn Rings/Pistons

Overhaul/Replace Engine

Excessive Cylinder Wear

Overhaul/Replace Engine

Plug Gap Bridged

Combustion Chamber Carbon Deposits

Clean Combustion Chamber

Blistered Electrode

Engine Overheating

Check Cooling System

Loose Spark Plugs

Clean/Torque Plugs

Over-Advanced Timing

Reset Timing

Wrong Plug Heat Range

Install Correct Plug

Melted Electrodes

Incorrect Timing

Reset Timing

Burned Valves

Replace Valves

Engine Overheating

Check Cooling System

Wrong Plug Heat Range

Install Correct Plug

Engine Won't Start

Loose Connections

Check Connections

No Power

Check Fuses/Battery

Loose/Worn Timing Belt/Chain/Gears

Check Belt/Chain/Gears

Problem & Possible Cause

Action

Engine Runs Rough

Leaky/Clogged Fuel Injectors

Repair Fuel Injectors

Leaky/Clogged Fuel Lines

Repair Fuel Lines

Clogged Fuel Filter

Replace Fuel Filter

Incorrect Timing

Reset Timing/Check Advance

Faulty Plugs/Wires

Replace Plugs/Wires

Uneven Compression

Overhaul/Replace Engine

Poor Acceleration

Incorrect Ignition Timing

Reset Timing

Leaky Valves

Check Compression

Component Failure

Spark Arcing

Replace Faulty Part

Defective Pick-Up Coil

Replace Pick-Up Coil

Defective Ignition Coil

Replace Ignition Coil

Defective Control Unit

Replace Control Unit

Ignition Diagnosis By Scope Pattern

All Firing Lines Abnormally High

Retarded Ignition Timing

Reset Ignition Timing

Lean Air/Fuel Mixture

Adjust Fuel Mixture

High Secondary Resistance

Repair Secondary Ignition

All Firing Lines Abnormally Low

Rich Air/Fuel Mixture

Adjust Air/Fuel Mixture

Arcing Coil Wire

Replace Coil Wire

Cracked Coil

Replace Coil

Low Coil Output

Replace Coil

Low Compression

Check/Repair Engine

Several High Firing Lines

Fuel Mixture Unbalanced

Check Fuel System

EGR Valve Stuck Open

Clean/Replace EGR Valve

High Plug Wire Resistance

Replace Plug Wire

Cracked/Broken Plugs

Replace Plugs

Intake Vacuum Leak

Repair Leak

Several Low Firing Lines

Fuel Mixture Unbalanced

Adjust Fuel Mixture

Plug Wires Arcing

Replace Plug Wires

Cracked Coil Arcing

Replace Coil

Uneven Compression

Check/Repair Engine

Faulty Spark Plugs

Replace Plugs

Cylinders Not Firing

Cracked Distributor Cap

Replace Cap

Shorted Plug Wires

Replace Plug Wires

Problem & Possible Cause

Action

Mechanical Engine Fault

Check/Repair Engine

Spark Plugs Fouled

Replace Plugs

Carbon Track in Distributor Cap

Replace Cap

Hard Starting

Defective Ignition Coil(s)

Replace Coil(s)

Fouled Spark Plugs

Replace Plugs

Incorrect Timing

Reset Ignition Timing

Problem & Possible Cause

Action

Cold Start Valve Inoperative

Test Cold Start Valve

Poor Vacuum/Electrical Connection

Repair Connections

Contaminated Fuel

Test Fuel for Water/Alcohol

Bad Fuel Pump Relay/Circuit

Test Relay/Wiring

Battery Voltage Low

Charge/Test Battery

Low Fuel Pressure

Test Press. Regulator/Pump

No Distributor Reference Pulse

Repair Ignition System

Coolant Temp. Sensor Defective

Test Temp. Sensor/Circuit

No Power To Injectors

Check Injector Fuse/Relay

Hard Starting

Defective Idle Air Control (IAC)

Test IAC and Circuit

EGR Valve Open

Test EGR Valve/Control Circuit

Restricted Fuel Lines

Inspect/Replace Fuel Lines

Poor MAP Sensor Signal

Test MAP Sensor/Circuit

Engine Stalls During Parking Maneuver

Check P.S. Press. Switch

Rough Idle

Dirty Fuel Injectors

Clean/Replace Injectors

Poor MAP Sensor Signal

Test MAP Sensor/Circuit

Intermittent Fuel Injector Operation

Check Harness Connectors

Erratic Vehicle Speed Sensor Inputs

Harness Too Close to Plug Wires

Poor O2 Sensor Signal

Test O2 Sensor/Circuit

Faulty PCV System

Check PCV Valve and Hoses

Poor Acceleration

Weak Fuel Pump

Replace Fuel Pump

Dirty Fuel Injectors

Clean/Replace Injectors

Excessive Intake Valve Deposits

Clean Intake System

Poor High Speed Operation

Problem & Possible Cause

Action

Low Fuel Pump Volume

Faulty Fuel Pump/Filter

Poor MAP Sensor Signal

Test Speed Sensor/Circuit

Acceleration Ping/Knock

Faulty EGR System

Check EGR Valve and Hoses

Poor Knock Sensor Signal

Test Knock Sensor/Circuit

Poor Baro Sensor Signal

Test Baro Sensor/Circuit

Improper Ignition Timing

Adjust Timing

Engine Overheating

Check Cooling System

Poor Quality Fuel

Use Different Fuel

Carbon Build-Up

Decarbon Engine