GM Service Manual Online
For 1990-2009 cars only

Checks

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DEFINITION: Fuel economy, as measured by actual road tests and several tanks of fuel, is noticeably lower than expected. Also, the economy is noticeably lower than it was on this vehicle at one time, as previously shown by actual road tests.

Preliminary Checks

    • Refer to Symptoms - Engine Controls .
    • Inspect air cleaner filter for dirt or being plugged.
    • Visually physically check: vacuum hoses for splits, kinks, and proper connections.
    • Inspect the driving habits of the owner.
    • Is the A/C ON full time, defroster mode ON?
    • Are the tires at the correct pressure?
    • Are excessively heavy loads being carried?
    • Is the acceleration too much, too often?
    • Suggest to the owner to fill the fuel tank and recheck the fuel economy.
    • Suggest to the driver to read the Important Facts on Fuel Economy in the Owner Manual.

Fuel System Checks

Inspect the fuel type and quality. Refer to Fuel Specific Gravity Testing .

Cooling System Checks

    • Inspect the engine coolant level.
    • Inspect the engine thermostats for always being open or for the wrong heat range.

Additional Checks

    • Inspect the transmission shift pattern.
    • Inspect the torque converter clutch (TCC) operation. When the TCC is commanded ON, a scan tool should indicate an RPM drop.
    • Inspect the Service Bulletins for control module software updates.
    • Check for dragging brakes.