Engine Controls (Troubleshooting & Diagnosis): Backfire

Backfire

Inspections	Actions
DEFINITION Fuel ignites in the intake manifold or in the exhaust system, making a loud popping noise.
Preliminary Inspections	Refer to Symptoms - Engine Controls . Search for bulletins. Inspect the engine control module (ECM) grounds for being clean, tight, and in the proper locations. Refer to Power and Grounding Component Views and Engine Controls Schematics in Wiring Systems.
Fuel System	Test for correct fuel pressure. Refer to Fuel System Diagnosis . Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis (without Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (with Special Tool) . Verify that each injector harness is connected to the correct injector or cylinder. Relocate injector harnesses as necessary.
Sensor/System	Inspect the air intake system and crankcase for air leaks. Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity.
Ignition System	Inspect for proper ignition voltage output with the J 26792 Spark Tester. See Special Tools . Refer to Electronic Ignition (EI) System Diagnosis . Inspect the coils for cracks, carbon tracking/arcing. Inspect spark plug boots for signs of arcing, cracks, carbon tracking, or damage. Remove spark plugs and inspect for the following conditions: Wet plugs Cracks Wear Improper gap Burned electrodes Heavy deposits Refer to Spark Plug Inspection . An improper spark plug gap will cause a driveability problem. Gap the spark plugs using a wire gage gap tool. Refer to Ignition System Specifications and Spark Plug Replacement . Determine the cause of the fouling before replacing the spark plugs. Soak the secondary ignition system with water from a spray bottle. Soaking the secondary ignition system may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as you apply the water. Monitor the Misfire Current Counters while driving the vehicle in the conditions that the misfire occurred. Inspect for loose ignition coil ground. Refer to Power and Grounding Component Views in Engine Electrical.
Engine Cooling System	Inspect the engine coolant level for being low. Refer to Loss of Coolant in Engine Cooling. Inspect the engine thermostat for proper operation and for the correct heat range. Refer to Thermostat Diagnosis in Engine Cooling.
Engine Mechanical	Inspect engine mechanical for the following conditions: Compression-Refer to Engine Compression Test in Engine Mechanical. Sticking or leaking valves Worn camshaft lobes Valve timing Worn rocker arms Broken valve springs Excessive oil in combustion chamber or leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical. For incorrect basic engine parts. Inspect the following components: The camshaft-Refer to Camshaft Cleaning and Inspection in Engine Mechanical. The cylinder heads-Refer to Cylinder Head Cleaning and Inspection in Engine Mechanical. The pistons, etc.-Refer to Piston, Connecting Rod, and Bearings Cleaning and Inspection in Engine Mechanical. Refer to Symptoms - Engine Mechanical in Engine Mechanical for diagnosis procedures.
Additional Inspections	Visually and physically inspect the vacuum hoses for splits, kinks, and proper connections and routing as shown on the Vehicle Emission Control Information label. Inspect the intake manifold and the exhaust manifold passages for casting flash. Refer to Intake Manifold Cleaning and Inspection or Exhaust Manifold Cleaning and Inspection - Left and Exhaust Manifold Cleaning and Inspection - Right in Engine Mechanical. Inspect the transmission torque converter clutch (TCC) operation. The scan tool should indicate an RPM drop when the TCC is commanded ON. Inspect the exhaust system for possible restrictions. Inspect the following conditions: The exhaust system for damaged or collapsed pipes The mufflers for heat distress or possible internal failure For possible plugged catalytic converters-Refer to Restricted Exhaust in Engine Exhaust. Electromagnetic interference (EMI) on the reference circuit can cause an engine miss condition. A scan tool can usually detect EMI by monitoring the engine RPM. A sudden increase in RPM with little change in actual engine RPM change may indicate that EMI is present. If a problem exists, inspect for high voltage components near the ignition control circuits.