GM Service Manual Online
For 1990-2009 cars
Makes 🢒 Chevrolet 🢒 2004 🢒 C-Series Medium Duty Truck C6/7/8 - Mexico 🢒 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. Also, 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 - Engine Controls .
    • Search for bulletins.
    • Check the powertrain control module (PCM) grounds for being clean, tight, and in the proper locations. Refer to Engine Controls Schematics .
    • Check the owners driving habits.
       - 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 check for dirt or for being restricted. Refer to Air Cleaner Element Replacement . Replace as necessary.

Fuel System
    • Check 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 .
    • Check the fuel injectors. Refer to Fuel Injector Solenoid Coil Test .
    • Check for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Check for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Check for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Check that each injector harness is connected to the correct injector and cylinder according to the firing order: 1-8-7-2-6-5-4-3. Relocate the injector harnesses as necessary.
    • Check for foreign material accumulation in the throttle bore, coking on the throttle valve, or on the throttle shaft.
    • Check items that cause an engine to run rich long term fuel trim near -13 percent. For a rich condition, refer to DTC P0132 or P0152 .

Sensor/System
    • Check the air intake system and crankcase for air leaks.
    • Check 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.
    • Check for proper calibration of the speedometer. Connect J 33431-B signal generator to the vehicle speed sensor (VSS) electrical connector. Turn ON the ignition with the tester ON and monitor the speedometer. The speedometer should indicate 86 km/h (54 mph).
    • Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity. Refer to Knock Sensor (KS) System Description .

Ignition System
    • Check for proper ignition voltage output with J 26792 spark tester.
    • Remove the spark plugs and check for the following conditions. Refer to Spark Plug Inspection :
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • An improper spark plug gap will cause a driveability problem. 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, coolant, or oil fouled. Refer to Spark Plug Inspection for diagnosis of fouled spark plugs.
    • Visually and physically inspect the secondary ignition for the following conditions:
       - 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 and listen for arcing or misfiring as you apply water.
    • Check for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .

Engine Cooling System
    • Check the engine coolant level for being low. Refer to Loss of Coolant in Engine Cooling.
    • Check the engine thermostat for proper operation and for the correct heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Check engine mechanical for the following conditions:
       - Check compression--Refer to Engine Compression Test in Engine Mechanical - 8.1L.
       - 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. Refer to Oil Consumption Diagnosis in Engine Mechanical - 8.1L.
    • Inspect the following components for incorrect basic engine parts. Refer to the appropriate procedures in Engine Mechanical for diagnosis procedures:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Additional Checks
    • Visually and physically check the vacuum hoses for splits, kinks, and proper connections and routing as shown on Vehicle Emission Control Information label.
    • Inspect the following components of the exhaust system for possible restrictions:
       - The exhaust system for damaged or collapsed pipes
       - The exhaust manifold for a collapsed inner wall
       - The mufflers for heat distress or possible internal failure
       - The three-way catalytic converters for possible plugged conditions--Compare the exhaust system back pressure on each side of engine. 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 EMI is present. Check for high voltage components, near ignition control circuits, if a problem exists.
    • Check the park neutral position (PNP) switch circuit.
    • Check the intake and the exhaust manifold passages for casting flash.
    • Check the brake system for dragging or improper operation. Refer to Brake Caliper Inspection in Disc Brakes. Ensure that the vehicle operator does not drive with a foot on the brake pedal.