GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC K Pickup - 4WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Poor Fuel Economy

Checks

Action

DEFINITION: Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the economy is noticeably lower than it once was on this vehicle at one time, as shown by an actual road test.

Preliminary Checks
    • Refer to Important Preliminary Checks in Symptoms .
    • Check the air filter for proper type and for dirt or restriction.
    • Check the vacuum hoses for leaks, restrictions and proper connections as shown on the vehicle emission control information (VECI) label.
    • Check the vehicle operators driving habits for the following items:
       - The A/C compressor clutch being engaged all the time (defroster).
       - 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
    • Check the fuel pressure. Refer to Fuel System Diagnosis .
    • Check for fuel system leakage.

Sensor Checks
    • Check 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.

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.
    • Check the Mass Air Flow sensor for proper operation.

Ignition System Checks
    • Remove and inspect the spark plugs for the following conditions:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • Check for an open ignition control (IC) circuit.
    • Check 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. A vehicle that operates normally on gasoline does not indicate a fully functional knock sensor / timing system.

    • Check for proper operation of the Knock Sensor system.
    • Check for proper CMP sensor adjustment and operation. The CMP Retard must be adjusted to 0 plus or minus 1 degree for proper engine operation on CNG fuel.