GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1999 🢒 Corvette 🢒 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, the fuel economy is noticeably lower than it was on this vehicle at one time, as previously shown by an actual road test.

Preliminary
    • Refer to Important Preliminary Checks Before Starting in Symptoms .
    • Search for bulletins.
    • Inspect the PCM grounds for being clean, tight, and in the proper locations. Refer to Engine Controls Schematics .
    • How are the owner's driving habits.
       - Is the A/C or the Defroster mode ON full time?
       - Are the tires at the correct pressure?
       - Is there excessively heavy loads being carried?
       - Is the acceleration rate too much, too often?
    • Remove the air filter element and inspect for dirt or for being restricted. Refer to Air Cleaner Element Replacement . Replace 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 Fuel System Specifications and Alcohol/Contaminants-in-Fuel Diagnosis .
    • Test for incorrect fuel pressure. 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 .
    • Inspect 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 the injector harnesses as necessary.
    • Inspect for foreign material accumulation in the throttle bore, coking on the throttle valve, or on the throttle shaft. Refer to Throttle Body Cleaning . Also inspect for throttle body tampering.
    • Inspect for items which can 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 .

Sensor/System
    • Inspect the air intake system and the 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 the speedometer. Connect 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 Gauge System Check 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 conditions:
       - Correct heat range; refer to Spark Plug Usage in Engine Electrical.
       - Wet plugs; refer to Spark Plug Visual Diagnosis in Engine Electrical.
       - Cracks; refer to Spark Plug Visual Diagnosis in Engine Electrical.
       - Wear; refer to Spark Plug Visual Diagnosis in Engine Electrical.
       - Improper gap; refer to Spark Plug Replacement in Engine Electrical.
       - Burned electrodes; refer to Spark Plug Visual Diagnosis in Engine Electrical.
       - Heavy deposits; refer to Spark Plug Visual Diagnosis in Engine Electrical.
    • An improper spark plug gap causes a driveability concern. Gap the spark plugs using a wire gauge gap tool (J 41319). Refer to Spark Plug Replacement in Engine Electrical.
    • Determine the cause of the fouling before replacing the spark plugs if spark plugs are gas, coolant or oil fouled. Refer to DTC P0172 Fuel Trim System Rich Bank 1 or DTC P0175 Fuel Trim System Rich Bank 2 for diagnosis of the rich condition. Refer to Spark Plug Visual Diagnosis in Engine Electrical for diagnosis of coolant or oil fouled spark plugs.
    • Visually/Physically inspect the secondary ignition for the following:
       - Ignition wires arcing to ground; refer to Spark Plug Wires Not Operating Properly in Engine Electrical.
       - Ignition wires for proper routing; refer to Spark Plug Wire Harness Replacement in Engine Electrical.
       - Ignition coils for cracks or carbon tracking
    • 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 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 the following components:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Refer to Engine Mechanical for the proper diagnosis procedures.

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 .
    • Test the Transmission Torque Converter Clutch (TCC) for proper operation. The scan tool should indicate an RPM drop, when the system commands the TCC ON. Refer to Functional Test in Automatic Transmission - 4L60 E.
    • Inspect the exhaust system for a possible restriction. Check the following components for the following conditions:
       - Inspect the exhaust system for damaged or collapsed pipes. Refer to Restricted Exhaust System Check .
       - Inspect the exhaust manifold for a collapsed inner wall.
       - Inspect the mufflers for heat distress or for possible internal failure. Refer to Restricted Exhaust System Check .
       - Inspect for possible plugged Three-Way catalytic converters by comparing the exhaust system back pressure on each side of the engine. Test the back pressure by removing the AIR check valves near the exhaust manifolds. 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 speed. A sudden increase in engine speed with little change in actual engine speed change, indicates that the EMI is present. Inspect for high voltage components near ignition control circuits if a problem exists.
    • Test PNP switch circuit . Refer to DTC P0705 Trans Range Switch Circuit or DTC P0706 Trans Range Switch Performance .
    • Inspect for faulty engine mounts. Refer to Engine Mount Replacement in Engine Mechanical-5.7L for inspection of the mounts.
    • Inspect the intake manifold and the exhaust manifold passages for casting flash. Refer to Intake Manifold Cleaning and Inspection or Exhaust Manifold Cleaning and Inspection in Engine Mechanical-5.7L.
    • Inspect the brake system for dragging or improper operation. Refer to Hydraulic Brake Diagnostic System Check in Hydraulic Brakes. Ensure that the vehicle operator does not drive with a foot on the brake pedal.