Diagnostic Information & Procedures: 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 Important Preliminary Inspections Before Starting in Symptoms - Engine Controls . Search for bulletins. Verify that the engine control module (ECM) grounds are clean, tight, and in the proper locations. Refer to Power and Grounding Component Views in Wiring Systems and Engine Controls Schematics .
Fuel System	Inspect for incorrect fuel pressure. Refer to Fuel System Diagnosis . Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis (with Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (without Special Tool) . Inspect the fuel injectors. Refer to Fuel Injector Coil Test , Fuel Injector Balance Test with Special Tool , or Fuel Injector Balance Test with Tech 2 . 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. Inspect the crankcase ventilation valve for proper operation. Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical. Inspect for an inaccurate speedometer Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity. Refer to Knock Sensor (KS) System Description .
Ignition System	Inspect for proper ignition voltage output with J 26792 Spark Tester. Remove spark plugs and inspect for the following: 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. Refer to Spark Plug Inspection . Gap the spark plugs using a wire gage gap tool. Refer to Spark Plug Replacement . Determine the cause of the fouling before replacing the spark plugs. Refer to Spark Plug Inspection for diagnosis. Visually and physically inspect secondary ignition for the following: Ignition wires arcing to ground Ignition coils arcing to ground 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. If a misfiring cylinder can be located, use the DTC P0300 table for diagnosis. Refer to DTC P0300 . Inspect for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .
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: Compression-Refer to Engine Compression Test in Engine Mechanical. Sticking or leaking valves Worn camshaft lobes Valve timing Bent push rods 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: The camshaft-Refer to Camshaft and Bearings 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. Inspect the transmission torque converter clutch (TCC) operation. The scan tool should indicate an RPM drop when the TCC is commanded ON. Refer to Torque Converter Diagnosis Procedure in Automatic Transmission - 4L60-E. Inspect the exhaust system for possible restrictions. Inspect the following: Inspect the exhaust system for damaged or collapsed pipes. Inspect the mufflers for heat distress or possible internal failure. Inspect 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. Inspect the park/neutral position (PNP) switch operation. Inspect for faulty motor mounts. Inspect the intake manifold and the exhaust manifold passages for casting flash