GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1999 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 Backfire

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DEFINITION : Fuel ignites in the intake manifold or in the exhaust system, making a loud popping noise.

Preliminary Checks
    • Refer to Important Preliminary Checks Before Starting in Symptoms .
    • Search for bulletins.
    • Inspect the PCM grounds for being clean, tight and in the proper locations. Refer to Engine Controls Schematics .

Fuel System
    • Test for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Test for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Test for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect that each injector harness is connected to the correct injector/cylinder according to the firing order (firing order: 1-8-7-2-6-5-4-3). Relocate injector harnesses as necessary.

Sensor/System
    • Test the Secondary Air Injection (AIR) system. Refer to DTC P0410 Secondary Air Injection (AIR) System .
    • Inspect the air intake system and crankcase for air leaks.
    • Test the crankcase ventilation valve for proper operation. Place a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve.
    • Verify proper calibration of speedometer. Connect Signal Generator J 33431-B to VSS electrical connector. Ignition ON, tester ON and monitor the speedometer. The speedometer should indicate 86 km/h (54 mph). Refer to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gauges and Console.
    • Use a scan tool in order to monitor the KS system for excessive spark retard activity. Refer to Knock Sensor (KS) System Description .

Ignition System
    • Test for proper ignition voltage output with spark tester J 26792 . Refer to Electronic Ignition (EI) System Diagnosis .
    • Inspect for an intermittent ignition system malfunction in the following circuits:
       - Ignition control circuit.
       - Power and ground to the ignition coils.
    • Remove spark plugs and inspect for the following:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits

Refer to Spark Plug Visual Diagnosis in Engine Electrical.

    • An improper spark plug gap causes a driveability condition. Gap the spark plugs using a wire gauge gap tool. Refer to Spark Plug Replacement in Engine Electrical.
    • Determine the cause of the fouling before replacing the spark plugs. If the spark plugs are gas fouled, refer to DTC P0172 Fuel Trim System Rich Bank 1 or DTC P0175 Fuel Trim System Rich Bank 2 . If the spark plugs are coolant or oil fouled, refer to Oil Consumption Diagnosis in Engine Mechanical-5.7L or Loss of Coolant in Engine Cooling.
    • Visually/Physically inspect secondary ignition for the following:
       - Ignition wires arcing to ground
       - Ignition coils arcing to ground
    • Wet down the secondary ignition system with water from a spray bottle. Wetting down the secondary ignition system may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as you apply the water.
    • 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 Cooling System Draining and Filling in Engine Cooling.
    • Test the engine thermostat for proper operation and heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Inspect engine mechanical for the following:
       - Test the engine compression. Refer to Engine Compression Test in Engine Mechanical-5.7L.
       - Sticking or leaking valves. Refer to Valve Train Diagnosis in Engine Mechanical-5.7L.
       - Worn camshaft lobes. Refer to Valve Train Diagnosis in Engine Mechanical-5.7L.
       - Valve timing. Refer to Valve Train Diagnosis in Engine Mechanical-5.7L.
       - Bent push rods. Refer to Valve Train Diagnosis in Engine Mechanical-5.7L.
       - Worn rocker arms. Refer to Valve Train Diagnosis in Engine Mechanical-5.7L.
       - Broken valve springs. Refer to Valve Train Diagnosis in Engine Mechanical-5.7L.
       - Excessive oil in combustion chamber-Leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical-5.7L.
    • For incorrect basic engine parts. Inspect the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Refer to Engine Mechanical-5.7L for diagnosis procedures.

Additional Checks
    • Visually (physically) Inspect the vacuum hoses for splits, kinks, and proper connections and routing as shown on Vehicle Emission Control Information label.
    • Inspect the intake manifold and the exhaust manifold passages for casting flash. Refer to Exhaust Manifold Cleaning and Inspection and Intake Manifold Clean and Inspect in Engine Mechanical-5.7L.
    • Test the Transmission Torque Converter Clutch (TCC) operation. The scan tool should indicate an RPM drop when the TCC is commanded ON. Refer to Functional Test in Automatic Transmission 4L60-E.
    • Inspect the exhaust system for possible restrictions. Inspect the following:
       - Inspect the exhaust system for damaged or collapsed pipes. Refer to Exhaust System Inspection in Engine Exhaust.
       - Inspect the exhaust manifold for a collapsed inner wall. Refer to Exhaust Manifold Cleaning and Inspection in Engine Exhaust.
       - Inspect the mufflers for heat distress or possible internal failure. Refer to Internal Muffler Rattles/Loss of Power in Engine Exhaust.
       - Inspect for possible plugged Three-Way catalytic converters by comparing exhaust system back pressure on each side of engine. Test back pressure by removing AIR check valves near exhaust manifolds or the Heated Oxygen sensors. Refer to Restricted Exhaust System Check .
    • 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. Inspect for high voltage components near Ignition Control circuits if a condition exists.
    • Test the PNP switch circuit. Refer to Park/Neutral Position Switch Diagnosis .
    • Inspect for faulty engine mounts. Refer to Engine Mount Inspection in Engine Mechanical-5.7L.