For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provide an overview of each diagnostic category.

### Circuit/System Description

This procedure is used for emission systems not equipped with P0300. An engine misfire is variations in the crankshaft rotation speed due to a cylinder misfire event. A misfire rate that is high enough can cause 3-way catalytic converter damage.

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

• | Engine Control Module Connector End Views

• | Engine Controls Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

• | Scan Tool Data List

• | Scan Tool Data Definitions

• | Scan Tool Output Controls

#### Special Tools Required

J 26792 Spark Tester

### Circuit/System Verification

1. Engine idling at the normal operating temperature.
2. Verify there is no abnormal engine noise.

⇒ | If there is an abnormal engine noise, refer to Symptoms - Engine Mechanical .

3. Verify the following DTCs are not set: P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0335, P0336, P0337, P0351, or P0352

⇒ | If a DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

4. Engine idling, disconnect one fuel injector connector at a time to isolate the misfiring cylinder. The engine speed should change when an injector is disconnected.
5. If the vehicle passes the Circuit/System Verification Test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records List.

### Circuit/System Testing

1. Verify that the following conditions do not exist:

• | Vacuum hose splits, kinks, and incorrect connections

• | Engine vacuum leaks

• | Crankcase ventilation system for vacuum leaks

• | Fuel pressure that is too low or too high--Refer to Fuel System Diagnosis .

• | Contaminated Fuel--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

• | Exhaust system restrictions

⇒ | If you find any of the above conditions, repair as necessary.

2. Ignition OFF, disconnect the spark plug wire from the misfiring cylinder.
3. Install the J 26792 to the boot of a spark plug wire and a good ground.

Important: An erratic or weak spark is considered a no spark condition.

4. Attempt to start the engine and observe the J 26792 . The spark tester should spark.

⇒ | If there is no spark, test the spark plug wire for less than 1,000 ohms per cm (per foot). Replace the spark plug wire if more than the specified value. If the spark plug wire test normal, refer to Electronic Ignition (EI) System Diagnosis for diagnosis of the ignition coil.

5. Ignition OFF, remove the spark plug from the misfiring cylinder. Verify that the following conditions do not exist with the spark plug:

• | Gas, coolant, or oil fouled

• | Cracked, worn, incorrectly gapped

⇒ | If there is a condition with the spark plug, replace the spark plug.

6. Exchange the suspected spark plug with another cylinder that is operating correctly.
7. Engine idling, disconnect the injector that corresponds to the misfiring cylinder. The engine speed should change.

⇒ | If the engine speed changes, replace the spark plug.

⇒ | If the engine speed does not change, test the fuel injector. Refer to Fuel Injector Solenoid Coil Test .

8. If all conditions test normal, test or inspect for an engine mechanical condition, refer to Symptoms - Engine Mechanical .

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Spark Plug Wire Replacement

• | Spark Plug Replacement