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.

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

Special Tools Required

J 26792 HEI Spark Tester

1. Engine running at the normal operating temperature. Verify there are no abnormal engine noise.

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

2. Verify that DTC P0201, P0202, P0203, P0204, P0336, P0337, P0351, or P0352 is not set.

⇒	If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

3. Engine running, disconnect one fuel injector connector at a time to isolate the misfiring cylinder. The engine speed should change when an injector is disconnected.
4. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

1. Verify 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 Spark Tester 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 31 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 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 running, 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 .

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.