GM Service Manual Online
For 1990-2009 cars only

Checks

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Definition: Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the economy is noticeably lower than previous, as shown by an actual road test.

Preliminary Checks

    • Refer to Symptoms .
    • Inspect the air cleaner filter for dirt or being plugged.
    • Visually inspect the vacuum hoses for splits, kinks, and proper connections as shown on the Vehicle Emission Control Information label.
    • Verify the owner's driving habits for the following items:
       - The A/C ON full time - Defroster mode ON?
       - The tires for being at the correct air pressure.
       - Excessively heavy loads being carried?
       - The acceleration too much, too often?
    • Suggest to the owner to fill the fuel tank and to recheck the fuel economy.
    • Suggest to the driver to refer to Important Facts on Fuel Economy in the owner's manual.

Fuel System Checks

    • Verify the fuel type, quality, and alcohol content.
    • Test the fuel pressure. Refer to Fuel System Diagnosis .

Ignition System Checks

    • Inspect the spark plugs. Remove the spark plugs and inspect for the following conditions:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • Inspect for an open Ignition Control (IC) circuit.
    • Inspect the ignition wires for the following items:
       - Cracking
       - Hardness
       - Proper connections
    • Test the Knock Sensor (KS) system operation. Refer to DTC P0327 Knock Sensor (KS) Circuit .

Cooling System Checks

    • Inspect the engine coolant level.
    • Inspect the engine thermostat for always being open or for the wrong heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Additional Checks

    • Test the transmission shift pattern. Refer to Harsh Shifts in Automatic Transmission - 4L60-E.
    • Test for Torque Converter Clutch (TCC) operation. Refer to Torque Converter Clutch Diagnosis in Automatic Transmission - 4L60-E.
    • Test for proper calibration of the speedometer.
    • Test for dragging brakes. Refer to the Hydraulic Brake section.