GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Geo 🢒 1997 🢒 Prizm 🢒 Engine 🢒 Engine Controls - 1.6L 🢒 Poor Fuel Economy

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 was on this vehicle at one time, as previously shown by an actual road test.

Preliminary Checks
    • Refer to Symptoms.
    • Check the air cleaner element (filter) for dirt or restrictions.
    • Visually/physically check the vacuum hoses for splits, kinks, and proper connections as shown on Vehicle Emission Control Information label.
    • Check the owner's driving habits for any of the following:
       - The A/C system (defroster mode) is ON all the time.
       - The tires are kept properly inflated.
       - Heavy loads are frequently being carried.
       - Heavy acceleration and high speeds are common.
       - Frequent short trips.
       - Prolonged periods of idling.
       - Driving on unpaved, dirt, or rough roads.
    • Suggest that the owner refill the fuel tank and recheck the fuel economy.

Fuel System Checks
    • Check the type, quality and alcohol content of the fuel. Oxygenated fuels have lower energy and may deliver reduced fuel economy. Refer to Fuel System Specifications Fuel System Specifications and Alcohol/Contaminants-in-Fuel Diagnosis.
    • Check for the correct fuel pressure. Refer to Fuel System Diagnosis.

Ignition System Checks
    • Check the spark plugs for any of the following conditions:
       - Wet (fuel fouled) plugs.
       - A terminal screw that is bent or loose (rotate and pull).
       - A ceramic insulator that has cracks, carbon tracking or deposits (red or black dust)
       - A tip insulator that has cracks, carbon tracking, deposits (oil, carbon, glazing, fuel additive), or is loose (rattles).
       - A center electrode that is loose (wiggles side to side)
       - Incorrect spark plug gap
       - Platinum pads -- missing, wear, gap size (too large or too small)
    • Check the ignition wires for cracking, chafing, and loose connections.
    • Observe the Spark parameter on the scan tool at idle, while operating the rear defogger, A/C system, or headlights. If the scan tool display remains fixed at 10 degrees check for a short to ground in the Diagnostic Request circuit from PCM terminal C2-15 to the underhood diagnostic link connector.

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 clutch adjustment in M/T equipped vehicles.
    • Check the transmission shift pattern and the TCC operation in A/T equipped vehicles.
    • Check for dragging brakes.
    • Inspect for the correct operation of the exhaust gas recirculation (EGR) system. An inoperative EGR system can cause excessive detonation that results in retarded ignition timing. Excessive EGR flow can cause incorrect air fuel ratios resulting in inefficient combustion.. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis.
    • Check for uneven or premature tire wear. A suspension misalignment can cause a loss of fuel economy.
    • Check that the tires and wheels are similar to OEM. Wide performance tires create more drag and taller tires can cause odometer discrepancies.
    • Check for the speedometer for proper calibration.
    • Check for a green engine. Fuel economy may not be near expected levels until the engine has had 3 to 5 thousand miles to break in.
    • Check for add-on equipment like luggage racks and carriers. Wind resistance can lower fuel economy.
    • Check the Service Bulletins for any updates or information.