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For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1998 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 5.7L 🢒 Poor Fuel Economy

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

Preliminary
    • Refer to Symptoms , Important Preliminary Checks Before Starting.
    • Search for bulletins.
    • Verify the PCM grounds for being clean, tight, and in the proper locations. Refer to Ground Distribution Schematics in Wiring Systems.
    • Document the owners driving habits.
    • Is the A/C ON or the Defroster mode ON full time?
       - Are the tires at the correct pressure? Refer to Tire Inflation Pressure Specifications in Maintenance and Lubrication.
       - Is there excessively heavy loads being carried?
       - Is the acceleration rate too much, too often?

Fuel System
    • Remove the air filter element and inspect for dirt or for being restricted. Refer to Air Cleaner Element Replacement . Replace as necessary.
    • Perform the injector balance test. Refer to Fuel Injector Balance Test for procedure.
    • Test for a low fuel pressure condition. Refer to Fuel System Diagnosis .
    • Test for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Test for a contaminated fuel condition Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Verify that each injector harness is connected to the correct injector/cylinder according to the firing order (firing order: 1-8-7-2-6-5-4-3). Relocate injector harnesses as necessary.
    • Test for injectors that are shorted internally. Compare the injector resistance. Injector resistance should be within one ohm of each other. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) .
    • Inspect for foreign material accumulation in the throttle bore, coking on the throttle valve, or on the throttle shaft. Also inspect for throttle body tampering.
    • Inspect items that cause an engine to run rich (long term fuel trim near -13%). For a rich condition, refer to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 and DTC P0152 HO2S Circuit High Voltage Bank 2 Sensor 1 .
    • Inspect items that can cause an engine to run lean (long term fuel trim near 23%). For a Lean condition, refer to DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1 and DTC P0151 HO2S Circuit Low Voltage Bank 2 Sensor 1 .

Sensor/System
    • Inspect the air intake system and crankcase for air leaks.
    • Test 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 not, replace the valve.
    • Test for proper calibration of speedometer. Connect the Signal Generator J 33431-B to the VSS electrical connector. Turn ON the ignition with the tester ON and monitor the speedometer. The speedometer should indicate 86 km/h (54 mph). If not, refer to Speedometer Inaccurate (J 38522 Available) in Instrument Panel, Gauges, 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
    • Test for proper ignition voltage output with spark tester J 26792 .
    • Remove the spark plugs and inspect for the following:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • An improper spark plug gap will cause a driveability condition. Do the spark plug gapping using a wire gauge gap tool (J 41319). Refer to Spark Plug Replacement. Determine the cause of the fouling before replacing the spark plugs if spark plugs are gas or oil fouled. Refer to DTC P0172 Fuel Trim System Rich Bank 1 and DTC P0175 Fuel Trim System Rich Bank 2 for diagnosis of the rich condition or Oil Consumption Diagnosis in Engine Mechanical 5.7L for diagnosis of the oil fouling condition.
    • Visually/Physically inspect the secondary ignition for the following:
       - Ignition wires arcing to ground
       - Ignition wires for proper routing
    • Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as you apply water.
        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.
    • Test the engine thermostat for proper operation and for the correct heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Inspect engine mechanical for the following:
       - Test compression. Refer to Engine Compression Test in Engine Mechanical 5.7L.
       - Sticking or leaking valves. Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Worn camshaft lobe(s). Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Valve timing. Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Bent push rods. Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Worn rocker arms. Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Broken valve springs. Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Excessive oil in combustion chamber-Leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical 5.7L.
    • For incorrect basic engine parts. Inspect for the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Additional Checks
    • Visually (physically) inspect the vacuum hoses for splits, kinks, and proper connections and routing as shown on Vehicle Emission Control Information label. Refer to Emission Hose Routing Diagram .
    • Verify the TCC operation. The scan tool should indicate a RPM drop, when the system commands the TCC ON. Refer to Torque Converter Clutch (TCC) Diagnosis in Automatic Transmission.
    • Inspect the exhaust system for a possible restriction. Inspect for the following:
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the exhaust manifold for a collapsed inner wall.
       - Inspect the mufflers for heat distress or possible internal failure.
       - Inspect for possible plugged Three-Way catalytic converters. Refer to Restricted Exhaust System Check .
    • 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 EMI is present. Inspect routing of secondary ignition wires, high voltage components (Near Ignition Control circuits) if a condition exists.
    • Test PNP switch circuit. Refer to Park/Neutral Position Switch Diagnosis .
    • Inspect for faulty motor mounts.
    • Inspect the intake and the exhaust manifold passages for casting flash. Refer to Intake Manifold Clean and Inspect and Exhaust Manifold Cleaning and Inspection in Engine Mechanical 5.7L.