GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC Suburban - 4WD 🢒 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.

Preliminary Checks
    • Refer to Symptoms .
    • Check the air cleaner element (filter) for dirt or being plugged.
    • Visually (physically) check the vacuum hoses for splits, kinks, and proper connections as shown on the Vehicle Emission Control Information label.
    • Check 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?
       - Agressive driving habits.
    • 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
    • Check the fuel type, quality, and alcohol content. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Check the fuel pressure. Refer to Fuel System Diagnosis .

Ignition System Checks
    • Check the spark plugs. Remove the spark plugs and check for the following:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • Check for an open Ignition Control (IC) circuit.
    • Check the ignition wires for the following items:
       - Cracking
       - Hardness
       - Proper connections
    • Check the Knock Sensor (KS) system operation.

Cooling System Checks
    • Check the engine coolant level.
    • Check the engine thermostat for always being open or for the wrong heat range.

Additional Checks
    • Check the transmission shift pattern. Refer to Shift Speed in Automatic Transmission 4L60E.
    • Check for Torque Converter Clutch (TCC) operation. Refer to Functional Test (4L60E) or Functional Test (4L80E) in Automatic Transmission.
    • Check for proper calibration of the speedometer.
    • Check for dragging brakes.