GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2002 🢒 Grand Prix 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 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:
       - Clip the J 26792 spark tester to engine ground.
       - 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.
       - Connect another spark plug wire between the remaining coil tower on the coil being tested and ground.
       - Crank the engine while observing the spark tester. A crisp blue spark should be observed.
       - 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)

   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.

Engine Mechanical
  • Test and inspect engine mechanical:
   • 

Inspect the compression

   • 

Sticking or leaking valves

   • 

Worn camshaft lobes

   • 

Valve timing

   • 

Bent push rods

   • 

Worn rocker arms.

   • 

Broken valve springs

   • 

Excessive oil in combustion chamber-Leaking valve seals

   • 

Low cylinder compression

    • 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 Transaxle Diagnosis.
    • 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.