GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2004 🢒 GTO 🢒 Engine 🢒 Engine Controls - 5.7L 🢒 Poor Fuel Economy

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DEFINITION : The fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the fuel economy is noticeably lower than previous fuel economy on this vehicle, as previously shown by an actual road test.

Preliminary
    • Refer to Important Preliminary Checks Before Starting in Symptoms - Engine Controls .
    • Search for bulletins.
    • Inspect the powertrain control module (PCM) grounds. Ensure that they are clean, tight, and in the proper locations. Refer to Engine Controls Schematics .
    • Check the owner's driving habits.
       - Is the A/C ON or the Defroster mode ON full time?
       - Are the tires at the correct pressure?
       - Are excessively heavy loads being carried?
       - Is the acceleration rate too much, too often?
    • Remove the air filter element. Inspect for dirt or for restrictions. Refer to Air Cleaner Element Replacement . Replace the element as necessary.

Fuel System
    • Inspect the type, quality, and alcohol content of the fuel. Oxygenated fuels have lower energy and may deliver reduced fuel economy. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect the fuel injectors. Refer to Fuel Injector Solenoid Coil Test .
    • Inspect for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Ensure that each injector harness is connected to the correct injector/cylinder according to the firing order. The firing order is, 1-8-7-2-6-5-4-3. Relocate the injector harnesses as necessary.
    • Inspect for foreign material accumulation in the throttle bore. Inspect for deposits on the throttle valve or on the throttle shaft. Also inspect for throttle body tampering. Refer to Throttle Body Cleaning .
    • Inspect for conditions that cause an engine to run rich, with a long term fuel trim near -13 percent. For a rich condition, refer to DTC P0132 or P0152 .

Sensor/System
    • Inspect the air intake system and the crankcase for air leaks.
    • Inspect the crankcase ventilation valve for proper operation. Place a finger over the inlet hole in the valve end several times. The valve should snap back. If the valve does not snap back, replace the valve.
    • Check the speedometer for proper calibration. Connect the Signal Generator J 33431-B to the VSS electrical connector. Turn ON the ignition, with the tester ON. Monitor the speedometer. The speedometer should indicate 86 km/h (54 mph). If not, refer to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gages and Console.
    • Use a scan tool in order to monitor the KS system for excessive spark retard activity. Refer to Knock Sensor (KS) System Description .

Ignition System
    • Inspect for proper ignition voltage output with spark tester J 26792 . Refer to Electronic Ignition (EI) System Diagnosis .
    • Remove the spark plugs. Inspect for the following conditions:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
       - Refer to Spark Plug Inspection .
    • An improper spark plug gap will cause a driveability condition. Gap the spark plugs using a wire gauge gap tool. Refer to Spark Plug Replacement .
    • Determine the cause of the fouling before replacing the spark plugs if the spark plugs are gas-fouled, coolant-fouled or oil-fouled. Refer to DTC P0172 or P0175 for diagnosis of the rich condition. Refer to Spark Plug Inspection for diagnosis of the coolant-fouled or the oil-fouled spark plugs.
    • Inspect the secondary ignition for the following conditions:
       - Ignition wires arcing to ground
       - Ignition wires for proper routing
    • Wetting the secondary ignition system with water from a spray bottle may help to locate damaged or deteriorated components. When you apply the water, look and listen for arcing or misfiring.
    • Inspect for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .

Engine Cooling System
    • Inspect the engine coolant level for being low. Refer to Cooling System Draining and Filling in Engine Cooling.
    • Inspect the engine thermostat for proper operation and heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Inspect the engine mechanical components for the following conditions:
       - Inspect 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
       - For specific symptoms, refer to Symptoms - Engine Mechanical in Engine Mechanical-5.7L.
       - Excessive oil in combustion chamber due to leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical.
    • Inspect for incorrect basic engine parts. Inspect the following components:
       - The camshaft
       - The cylinder heads
       - The pistons

Additional Checks
    • Inspect the vacuum hoses for splits, for kinks, and for proper connections and routing as shown on Vehicle Emission Control Information label.
    • Inspect the transmission torque converter clutch (TCC) operation. When the system commands the TCC ON, the scan tool should indicate a RPM drop. Refer to Diagnostic Starting Point - Automatic Transmission in Automatic Transmission-4L60-E.
    • Inspect the exhaust system for a possible restriction. Inspect for the following conditions:
       - Inspect the exhaust system for damaged or collapsed pipes. Refer to Restricted Exhaust in Engine Exhaust.
       - Inspect the exhaust manifold for a collapsed inner wall. Refer to Exhaust Manifold Cleaning and Inspection in Engine Exhaust.
       - Inspect the mufflers for heat distress or possible internal failure
       - Inspect for possible plugged 3-way catalytic converters by comparing the exhaust system back pressure on each side of the engine. Inspect the back pressure by removing the AIR check valves near the exhaust manifolds. Refer to Restricted Exhaust in Engine Exhaust.
    • Electromagnetic Interference (EMI) on the reference circuit can cause an engine miss condition. A scan tool can usually detect EMI by monitoring the engine RPM. A sudden increase in RPM, with little change in actual engine RPM change indicates that EMI is present. If a condition exists, inspect for high voltage components near the ignition control circuits.
    • Inspect the PNP switch circuit. Refer to Park/Neutral Position Switch Adjustment in Automatic Transmission-4L60-E.
    • Inspect for faulty motor mounts. Refer to Engine Mount Inspection in Engine Mechanical-5.7L.
    • Inspect the intake and the exhaust manifold passages for casting flash. Refer to Exhaust Manifold Cleaning and Inspection in Engine Mechanical.
    • Inspect the brake system for dragging or improper operation. Refer to Symptoms - Hydraulic Brakes in Hydraulic Brakes.