GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2005 🢒 Century 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 Poor Fuel Economy

Inspection/Test

Action

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

Preliminary
    • Perform the Diagnostic System Check - Vehicle in Vehicle DTC Information
    • Refer to Intermittent Conditions before starting.
    • Search for bulletins.
    • Observe the owners driving habits.
       - Is the A/C ON full time, defroster mode ON?
       - Are tires at correct pressure?
       - Are tires and wheels the correct size?
       - Are excessively heavy loads being carried?
       - Is acceleration too much, too often?

Fuel System
    • Test the fuel system circuits for proper operation. Refer to Fuel Pump Electrical Circuit Diagnosis .
    • Test for low fuel pressure. Refer to Fuel System Diagnosis .
    • Test for faulty fuel injectors. Refer to Fuel Injector Balance Test with Special Tool , Fuel Injector Balance Test with Tech 2 , Fuel Injector Solenoid Coil Test test procedures.
    • Inspect for fuel contamination. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Ensure that each injector harness is connected to the correct injector/cylinder.
    • Inspect for fuel in the pressure regulator vacuum hose.

Sensor/System
    • Observe the 24X crank sensor and CMP sensor signal present parameters on the scan tool. If both are not responding, inspect the sensor feed circuit. Both sensors use a common feed circuit.
    • Inspect the air intake system and crankcase for air leaks.
        Inspect the air intake ducts for being collapsed, damaged areas, looseness, improper installation, or leaking, especially between the mass air flow (MAF) sensor and the throttle body.
    • Inspect for proper operation of the speedometer. Refer to Symptoms - Instrument Panel, Gages and Console in Instrument Panel, Gages, and Console.

Ignition System
    • Inspect for proper ignition voltage output using the following steps:
        1. Attach the J 26792 Spark Tester to engine ground.
        2. Connect the spark plug end of the spark plug wire to the J 26792 . Connect the other end of the spark plug wire to the coil being tested.
        3. Connect the spark plug end of the companion spark plug wire to ground.

The companion spark plug wire is the wire attached to the corresponding coil tower.

        4. Crank the engine while observing the J 26792 . A spark should be observed.
        5. Repeat the above steps for each coil.
    • If spark is not present at the coils, inspect for the following conditions:
       - Coils--Cracks, carbon tracking/arcing, or a resistance value outside the specified range.

Coil Resistance
5,000-8,000 ohms

       - Spark Plug wires--signs of arcing, cross firing, cracks, carbon tracking, plug boot damage, pinched, improper routing, or a resistance value outside the specified range.

Spark Plug Wire Resistance
3,000 ohms per foot (9,842 ohms per meter).

    Important: : Spraying the secondary ignition wires with a light mist of water may help locate an intermittent problem. Ignition voltage arcs to ground when a secondary component is faulty.

       - Defective ignition module
       - Ignition system wiring--Loose ignition module feed or ground connection, or damaged system wiring.
    • Remove spark plugs and inspect for the following conditions:
       - Fouled plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned or damaged electrodes
       - Improper heat range or reach
    • If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs. Refer to Spark Plug Inspection .

Engine Cooling System
    • Inspect the engine coolant level for correct level.
    • Inspect for incorrect or faulty engine thermostat. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Inspect the engine for the following conditions:
       - Test the compression--Refer to Engine Compression Test in Engine Mechanical.
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber--Leaking valve seals
    • Inspect for incorrect engine parts, such as the following basic components:
       - The camshaft
       - The cylinder heads
       - The pistons, etc.
    • Refer to Symptoms - Engine Mechanical in Engine Mechanical.

Additional
    • Test torque converter clutch (TCC) operation. Refer to Torque Converter Diagnosis in Automatic Transaxle.
    • Inspect exhaust system for possible restriction:
       - Refer to Restricted Exhaust in Engine Exhaust.
       - Inspect exhaust system for damaged or collapsed pipes.
       - Inspect mufflers for heat distress or possible internal failure.
       - For possible plugged catalytic converter.