GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC C-Series Medium Duty Truck B7 🢒 Body and Accessories 🢒 Wiring Systems 🢒 Lack of Power, Sluggishness, or Sponginess

Checks

Action

DEFINITION: Engine delivers less than expected power. Little or no increase in speed when the accelerator pedal is pushed down part way.

Preliminary Checks
    • Refer to Symptoms , Important Preliminary Checks Before Starting.
    • Search for bulletins.
    • Check the PCM grounds for being clean, tight and in their proper locations. Refer to Wiring Repairs in Wiring Systems.
    • Remove the air filter element and check for dirt or for being restricted. Refer to Air Cleaner Element Replacement and replace as necessary.

Fuel System Checks
    • Check both injector fuses for being open. An open injector fuse causes four ignition coils and four injectors not to operate. Replace the fuse. Inspect the ignition coil circuits and the injector circuits for an intermittent short to ground.
    • 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 the 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 the 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 EGR valve for being open or partially open all the time. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis .
    • Check the generator output voltage. Refer to Engine Electrical.
    • 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 both injector fuses for being open. An open injector fuse causes four ignition coils and four injectors not to operate. Replace the fuse. Inspect the ignition coil circuit and the injector circuits for an intermittent short to ground.
    • Wet down the secondary ignition system with water from a spray bottle. Wetting the secondary ignition system may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as water is applied.
    • 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. Refer to Maintenance Items in Maintenance and Lubrication. 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 Engine Mechanical for diagnosis of the oil fouling condition.
    • Check for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .
    • Check the spark plugs for proper heat range. Refer to Maintenance Items in Maintenance and Lubrication.

Engine Mechanical
    • Ensure that the engine coolant temperature is not above 130°C (266°F). This condition causes the PCM to operate in Engine Coolant Over temperature-Fuel Disabled Mode. While in Engine Coolant Over temperature-Fuel Disabled Mode, the PCM will turn fuel off to four cylinders at a time to keep engine temperatures from reaching damaging levels. The system perceives the Engine Coolant Over temperature-Fuel Disabled Mode as a lack of power, miss, or rough idle. If the vehicle operates in Engine Coolant Over temperature-Fuel Disabled Mode, refer to Engine Cooling for diagnosis.
    • Excessive oil in combustion chamber-Leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical.
    • Low cylinder compression. Refer to Engine Compression Test in Engine Mechanical.
    • For incorrect basic engine parts. Inspect the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Refer to Engine Mechanical.

Additional Checks

Check the exhaust system for possible restrictions. 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 catalytic converters by comparing exhaust system back pressure on each side of engine. Check back pressure by removing Heated Oxygen Sensor (HO2S) near exhaust manifolds. Refer to Restricted Exhaust System Check .