GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Optra 🢒 Engine 🢒 Engine Controls - 1.6L (L91) 🢒 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 - Engine Controls .
    • Search for bulletins.
    • Verify that the engine control module (ECM) grounds are clean, tight, and in the correct locations.

Fuel System
    • Test for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Test for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Verify that the fuel injector harness is connected to the correct injector/cylinder according to the firing order.
    • Test the fuel injectors. Refer to Fuel Injector Solenoid Coil Test .
    • Test or inspect items that can cause an engine to run lean. For a lean condition, refer to DTC P0171 .

Sensor/System
    • Inspect the air intake system and the crankcase for air leaks.
    • Test the crankcase ventilation valve.
    • 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 and DTC P0327 .

Ignition System
    • Test the ignition voltage output with the J 26792 Spark Tester. Refer to Electronic Ignition (EI) System Diagnosis .
    • Inspect for an intermittent ignition system malfunction in the ignition control circuit. Use the Snapshot feature on the scan tool to locate an intermittent failure.
    • Remove the spark plugs and inspect for the following conditions:
       - Incorrect heat range
       - Wet plugs
       - Cracks
       - Excessive wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
       - Refer to the following procedures:
    • An improper spark plug gap can cause a driveability concern. Refer to Ignition System Specifications .
    • If the spark plugs are fouled by gas, coolant, or oil, determine the cause before replacing the plugs. Refer to the following procedures:
       - For diagnosis of a rich condition, refer to DTC P0172 .
       - For diagnosis of coolant or oil fouled spark plugs, refer to Loss of Coolant in Engine Cooling or Symptoms - Engine Mechanical in Engine Mechanical - 1.6L (L91).
    • Inspect the spark plug wires for damage. Refer to Spark Plug Wire Inspection .
    • 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 the water is applied.
    • Test the crankshaft position (CKP) sensor for the correct resistance. The CKP sensor resistance may be out of range after a hot soak. The resistance should be between 460-620 ohms.
    • Inspect the ignition coil for cracks or carbon tracking.

Engine Cooling System
    • Inspect the engine coolant level. Refer to Cooling System Draining and Filling in Engine Cooling.
    • Test the engine thermostat. Verify that the thermostat is of the correct heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Inspect for the following engine mechanical conditions:
       - Incorrect compression
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Incorrect valve timing
       - Broken valve springs
       - Excessive oil in the combustion chamber or leaking valve seals
       - For more information, refer to the following procedures in Engine Mechanical - 1.6L (L91):
   •  Oil Leak Diagnosis
    • Inspect the following for incorrect basic engine parts:
       - The camshaft
       - The cylinder heads
       - The pistons, connecting rods, and bearings
       - Refer to the following procedures in Engine Mechanical - 1.6L (L91):

Additional Checks
    • Inspect the vacuum hoses for splits and kinks.
    • Test the transmission torque converter clutch (TCC). The scan tool should indicate an engine speed drop when the TCC is commanded ON.
    • Inspect the following exhaust system components:
       - The exhaust system for physical damage
       - The exhaust manifold for a collapsed inner wall
       - The mufflers for heat distress or possible internal failure
       - The three-way catalytic converter(s) for restrictions.
       - For more information, refer to Symptoms - Engine Exhaust in Engine Exhaust.
    • Electromagnetic interference (EMI) on the reference circuit can cause an engine misfire condition. You can usually detect EMI with a scan tool by monitoring the engine speed parameter. A sudden increase of the engine speed parameter with little change in actual engine speed may indicate that EMI is present. If a problem exists, inspect the high voltage components near the ignition control circuits.