GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2000 🢒 Century (China) 🢒 Engine 🢒 Engine Controls - 2.5L (LB8) and 3.0L (L46) 🢒 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 Intermittent Conditions .
    •  Search for bulletins.

Sensor/System
    •  Inspect for the following intermittent ignition system conditions:
       -  Intermittent 24X CKP 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
    •  Inspect for low fuel pressure. Refer to Fuel System Pressure Test .
    •  Inspect for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    •  Ensure each injector harness is connected to the correct injector/cylinder.

Ignition System
    •  Inspect 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 J 26792 spark tester. Leave the other end of the spark plug wire to the coil being tested.
       -  Connect one end of another spark plug wire to the other coil tower. Connect the other end of the spark plug wire to ground.
       -  Crank the engine while observing the J 26792 spark tester. A spark should be observed.
       -  Repeat the above steps for each coil.
    •  If adequate spark is not present at the coils, inspect for the following conditions:
       -  Coils--Cracks, carbon tracking/arcing or a resistance value outside the specified range.

Coil Resistance
5K-7K ohms (5,000-7,000 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
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 problem. 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 the spark plugs and inspect for the following:
       -  Fouled plugs
       -  Cracks
       -  Wear
       -  Improper gap
       -  Burned or damaged electrodes
       -  Improper heat range or reach
    •  If the spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.

Engine Mechanical

Check engine mechanical for the following:

    •  Check 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 Base Engine Misfire Diagnosis in Engine Mechanical.

Additional Checks
    • Inspect intake and exhaust manifolds for casting flash.
    • Inspect the TCC operation. Refer to Torque Converter Diagnosis in Automatic Transaxle-4T65-E.
    • Inspect the exhaust system for possible restriction:
       -  Inspect the exhaust system for damaged or collapsed pipes.
       -  Inspect the mufflers for heat distress or possible internal failure.
       - Inspect for possible plugged three-way catalytic converter. Refer to Exhaust Leakage - Not OBD II in Engine Exhaust and Restricted Exhaust System Check .