GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Express (CNG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 Poor Fuel Economy

Checks

Action

DEFINITION: Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the current fuel economy is noticeably lower than the vehicle's previous fuel economy, as shown by an actual road test.

Preliminary Checks
    • Refer to Important Preliminary Checks in Symptoms - Engine Controls .
    • Inspect the air filter for proper type and for dirt or restriction.
    • Inspect the vacuum hoses for leaks, restrictions and proper connections as shown on the vehicle emission control information (VECI) label.
    • Review the vehicle operators driving habits for the following items:
       - The A/C compressor clutch being engaged all the time
       - Excessive idling or stop and go driving
       - Proper tire inflation
       - Low CNG refill pressure -- check fill station pressure
       - Excessively heavy loads being carried
       - Too much acceleration, too often

Advise vehicle operator to fill the fuel tank and recheck fuel economy.

Fuel System Checks

       Important: The customer may comment that one fuel tank does not appear to fill. An inoperative high pressure lock-off (HPL) solenoid valve will allow a fuel tank to fill, but will not allow it to empty.

    • Test all of the high pressure lock-off (HPL) solenoids for proper operation. Test by performing the following:
        1. Disconnect the HPL solenoid power and ground connector at each CNG fuel tank except one.
        2. Operate the engine on CNG for at least 2 minutes while only one HPL solenoid is connected.
        3. Observe the fuel gage while the engine is operating.
        4. Repeat the above procedure for each HPL until all CNG fuel tanks have independently operated the engine.
        5. An HPL condition exists if any of the following symptoms are observed:
   • The fuel gage drops toward empty.
   • The engine switches over to gasoline operation (KL6).
   • The engine stalls (KL8).
        6. If an HPL condition exists, test for the following:
   • A closed HPL manual shut-off valve.
   • A lack of power or ground at the HPL connector when commanded ON.
   • An open or shorted HPL solenoid. Replace the HPL as necessary. Refer to High Pressure Lock-Off Solenoid Replacement .
    • Test the fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect for fuel system leakage.

Sensor Checks

Test the ECT sensor for proper operation by performing the following:

  1. Allow the engine temperature to reach ambient air temperature by leaving the engine OFF for several hours -- overnight for most accurate results.
  2. Turn the ignition ON while leaving the engine OFF.
  3. Using a scan tool compare the ECT and IAT sensors displayed temperature.

    4. The sensors should be within 3°C (5°F) of each other.

  4. Replace the ECT sensor or repair the ECT sensor circuit if the ECT is not near the expected value.

Ignition System Checks
    • Remove and inspect the spark plugs for the following conditions:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • Test for an open ignition control (IC) circuit.
    • Inspect the spark plug wires for the following:
       - Cracking
       - Hardness
       - Proper connections and routing

       Important:: More timing advance is required to operate the vehicle on CNG than on gasoline.

    • Test for proper operation of the Knock Sensor system.