GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 7.4L 🢒 Poor Fuel Economy

Checks

Action

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 .
    • Search for bulletins.
    • Check the PCM grounds for being clean, tight, and in the proper locations. Refer to Wiring Repairs in Wiring Systems.
    • Check the owners driving habits.
    • 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 check for dirt or for being restricted. Refer to Air Cleaner Element Replacement . Replace as necessary.

Fuel System
    • Perform the injector balance test. Refer to Sequential Multiport Fuel Injector Balance Test for procedure.
    • Check for a low fuel pressure condition. 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/cylinder according to the firing order (firing order: 1-8-4-3-6-5-7-2). Relocate injector harnesses as necessary.
    • Check 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 .
    • Check for foreign material accumulation in the throttle bore, coking on the throttle valve, or on the throttle shaft. Also inspect for throttle body tampering.
    • Check 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 .
    • Check 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
    • 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.
    • 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
    • Check for proper ignition voltage output with spark tester J 26792 .
    • Remove the spark plugs and check for the following:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • An improper spark plug gap will cause a driveability problem. 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 Fuel System Diagnosis for diagnosis of the rich condition or 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.
        Check for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .

Engine Cooling System
    • Check the engine coolant level for being low.
    • Check the engine thermostat for proper operation and for the correct heat range.

Engine Mechanical
    • Check engine mechanical for the following:
       - Check compression
       - Sticking or leaking valves
       - Worn camshaft lobe(s)
       - Valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber-Leaking valve seals
       - Low cylinder compression
    • For incorrect basic engine parts. Inspect for the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Refer to Engine Mechanical for diagnosis procedures.

Additional Checks
    • Visually (physically) check the vacuum hoses for splits, kinks, and proper connections and routing as shown on Vehicle Emission Control Information label.
    • Check the exhaust system for a possible restriction. Check 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 by comparing the exhaust system back pressure on each side of the engine. Check 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 RPM. A sudden increase in RPM with little change in actual engine RPM change, indicates EMI is present. Check routing of secondary ignition wires, high voltage components (Near Ignition Control circuits) if a problem exists.
    • Check PNP switch circuit . Refer to Park/Neutral Position Switch Diagnosis .
    • Check for faulty motor mounts. Refer to Engine Mount Inspection in Engine Mechanical for inspection of mounts.
    • Check the intake and the exhaust manifold passages for casting flash.
    • Check for proper calibration of the speedometer.
    • Check for dragging brakes. Refer to Section 5 of the appropriate service manual.