GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2003 🢒 Bonneville 🢒 Engine 🢒 Engine Electrical 🢒 Backfire

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

Preliminary Checks
    • Refer to Important Preliminary Checks.
    • Refer to Visual/Physical Checks.
    • Search for bulletins.

Sensor/System
    • Test for the following intermittent ignition system conditions:
       - Intermittent CKP 18X signal
       - Intermittent ignition feed circuit or sensor ground circuit to the crankshaft position sensor or the camshaft position sensor
       - Intermittent 3X reference signal
       - Intermittent camshaft position signal
    • Inspect the mass air flow (MAF) sensor conditions. Ensure that the MAF sensor honeycomb is not damaged.

Fuel System
    • Test for low fuel pressure. Refer to Fuel System Diagnosis .
    • Test for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Ensure each injector harness is connected to the correct injector/cylinder.

Ignition System
    • Test for proper ignition voltage output using the following steps:
        1. Clip the J 26792 spark tester to engine ground.
        2. Connect one end of a spark plug wire to the spark tester. Leave the other end of the spark plug wire connected to the coil being tested.
        3. Connect another spark plug wire between the remaining coil tower on the coil being tested and ground.
        4. Crank the engine while observing the spark tester. A crisp blue spark should be observed.
        5. Repeat the above steps for each coil.
    • If spark is not present at the coils, test for the following conditions:
       - Coils--Cracks, carbon tracking/arcing, or a secondary resistance value outside the specified range

Coil Resistance
5,000-8,000 ohms (5K-8K ohms)

       - Spark plug wires--Signs of arcing/cross firing, cracks, carbon tracking, plug boot damage, pinched, improper routing, or a resistance value outside the specified range.

Spark Plug Wire Resistance
9840 ohms per meter (3000 ohms per foot)

Spark Plug Wire Resistance Police
1,968 ohms per meter (600 ohms per foot)

   Important: Spraying the secondary ignition wires with a light mist of water may help locate an intermittent condition. Ignition components will arc to ground when a secondary component is faulty.

       - Defective module
       - Ignition system wiring--Loose ignition module feed or ground connection, or damaged system wiring
    • Remove spark plugs and inspect for the following:
       - Fouled plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned or damaged electrodes
       - Improper heat range or reach
    • If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs. Refer to Spark Plug Inspection .

Engine Mechanical
    • Test and inspect engine mechanical:
       - Inspect the compression, refer to Engine Compression Test in Engine Mechanical-3.8L.
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods, refer to Valve Rocker Arm and Push Rod Cleaning and Inspection in Engine Mechanical-3.8L.
       - Worn rocker arms. Refer to Valve Rocker Arm and Push Rod Cleaning and Inspection in Engine Mechanical-3.8L.
       - Broken valve springs, refer to Cylinder Head Cleaning and Inspection in Engine Mechanical-3.8L.
       - Excessive oil in combustion chamber-Leaking valve seals
    • For incorrect basic engine parts--Inspect the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.
    • Refer to Symptoms - Engine Mechanical in Engine Mechanical-3.8L for diagnosis procedures.

Additional Checks
    • Inspect Intake and exhaust manifolds for casting flash.
    • Test torque converter clutch (TCC) operation. Refer to Torque Converter Diagnosis in Automatic Transaxle--4T65E.
    • Test the exhaust system for possible restriction. Inspect the following:
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the mufflers for heat distress or possible internal failure.
       - For possible plugged 3-way catalytic converter--Refer to Restricted Exhaust in Engine Exhaust.