GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 TopKick C-Series (Conventional) C4/C5 🢒 Engine 🢒 Engine Controls - 8.1L 🢒 Poor Fuel Economy

Inspections

Action

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.
    • Verify that the powertrain control module (PCM) grounds are clean, tight, and in the proper locations. Refer to Power and Grounding Component Views in Wiring Systems and Engine Controls Schematics .
    • Verify the following information:
       - Is the A/C ON 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 a restriction. Refer to Air Cleaner Element Replacement . Replace as necessary.

Fuel System
    • Verify 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 .
    • Test the fuel injectors for correct operation. Refer to Fuel Injector Solenoid Coil Test and Fuel Injector Balance Test with Tech 2 .
    • Inspect for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Verify that each injector harness is connected to the correct injector and cylinder. 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.
    • Inspect the following that may cause the engine to run rich:

Notice: Refer to Heated Oxygen and Oxygen Sensor Notice in the Preface section.

       - Water intrusion in the HO2S connector
       - Engine oil contaminated by fuel
       - An evaporative emission (EVAP) canister purge condition
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - A leaking fuel pressure regulator--Refer to Fuel System Diagnosis .
       - Rich fuel injectors--Refer to Fuel System Diagnosis .
       - Inaccurate mass air flow (MAF) sensor--Refer to Scan Tool Data List .
       - Blockage on the inlet screen of the MAF sensor--Refer to Mass Airflow Sensor/Intake Air Temperature Sensor Replacement .
       - Vacuum hoses for splits, kinks, and proper connections
       - The air intake duct for being collapsed or restricted--Refer to Air Cleaner Outlet Front Duct Replacement .
       - The air filter for being dirty or restricted--Refer to Air Cleaner Element Replacement .

Sensor/System
    • Inspect the air intake system and 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 not, replace the valve.
    • Inspect for an inaccurate speedometer. Refer to Speedometer and/or Odometer Inaccurate or Inoperative in Instrument Panel, Gages and Console.
    • Monitor the knock sensor (KS) system for excessive spark retard activity with a scan tool. Refer to Knock Sensor (KS) System Description .

Ignition System
    • Test for proper ignition voltage output with the J 26792 Spark Tester.
        Remove the spark plugs and inspect 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 gage 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:
       - Spark plug wires arcing to ground
       - Spark plug 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.
    • 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 Loss of Coolant in Engine Cooling.
    • Inspect the engine thermostat for proper operation and for the correct heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Inspect the engine mechanical for the following conditions:
       - 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--Refer to Oil Consumption Diagnosis in Engine Mechanical .
    • 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
    • Visually and physically inspect 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
       - 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 the engine speed parameter on the scan tool with little change in actual engine RPM change indicates EMI is present. Inspect for high voltage components, near ignition control circuits, if a problem exists.
    • Inspect the intake and the exhaust manifold passages for casting flash.
    • Inspect for a malfunctioning exhaust brake valve. Refer to Symptoms - Engine Exhaust Brake in Engine Exhaust.
    • Inspect the brake system for dragging or improper operation. Ensure that the vehicle operator does not drive with a foot on the brake pedal.