GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Isuzu 🢒 2001 🢒 S/T Isuzu - 4WD 🢒 Engine 🢒 Engine Electrical 🢒 Poor Fuel Economy

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DEFINITION: Fuel economy, as measured by an actual road test is noticeably lower than expected.

Preliminary
    • Refer to Symptoms - Engine Controls .
    • Search for service bulletins.
    • Check owners driving habits.
       - Is A/C ON full time (Defroster mode ON)?
       - Are tires at correct pressure?
       - Are excessively heavy loads being carried?
       - Is acceleration too much, too often?
       - Is E85 ethanol fuel used? If so, suggest the owner perform an actual fuel mileage comparison using standard unleaded fuel. If the fuel mileage is correct with standard unleaded fuel, the fuel consumption is normal.

Fuel System
    • Check the fuel pressure. Refer to Fuel System Diagnosis .
    • Ensure each injector harness is connected to the correct injector/cylinder. Refer to Engine Controls Schematics .
    • Check items which can cause an engine to run rich--Long Term FT significantly in the negative range. Refer to Diagnostic Aids for DTC P0172 .
    • Check items that can cause an engine to run lean--Long Term FT significantly in the positive range. Refer to Diagnostic Aids for DTC P0171 .
    • Check for proper operation of the evaporative emission (EVAP) control system.

Sensor/System
    • Check the oxygen sensor (O2S 1). The O2S 1 should respond quickly to different throttle positions. If not, check the O2S 1 for silicon or other contaminants from fuel or the use of improper RTV sealant. The sensors may have a white powdery coating. Silicon contamination causes a high but false O2S 1 signal voltage--rich exhaust indication. The PCM will reduce the amount of fuel delivered to the engine, causing a severe driveability problem.
    • Check for proper operation of the MAP sensor.
    • Check for proper operation of the TP sensor.
    • Check for proper operation of the ECT sensor.

Ignition System
    • Check for proper ignition voltage output with the J 26792 spark tester or equivalent.
    • Check the spark plugs for the following conditions:
       - Wet or fuel fouled
       - Cracks
       - Wear
       - Improper spark plug gap
       - Burned electrodes or heavy deposits
    • If the spark plugs are fuel or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.
    • Check the ignition control module (ICM) for proper ground connection.
    • Check the ignition coils for cracks or carbon tracking.
    • Check the spark plug wires for proper resistance and condition.

Engine Cooling System
    • Check engine coolant level for being low. Refer to Loss of Coolant in Engine Cooling.
    • Check for incorrect or malfunctioning engine thermostat. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Check engine mechanical for the following:
       - 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
    • For incorrect basic engine parts. Inspect for the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.
    • Refer to Thermostat Diagnosis in Engine Mechanical for diagnosis procedures. To replace, refer to Engine Coolant Thermostat Replacement .

Additional Checks
    • Check TCC operation. Refer to Torque Converter Clutch in Transaxle Diagnosis.
    • Check exhaust system for possible restriction. Check for the following:
       - Inspect exhaust system for damaged or collapsed pipes.
       - Inspect mufflers for heat distress or possible internal failure.
       - For possible plugged three-way catalytic converter. Refer to Restricted Exhaust System Check .