GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 Sierra Classic - 2WD 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic Information 🢒 Lack of Power, Sluggishness, or Sponginess

Lack of Power, Sluggish or Spongy

Inspection/Tests

Action

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

Preliminary Inspections
    • Refer to Important Preliminary Inspections 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 and Engine Controls Schematics .
    • Remove the air filter element and inspect for dirt or for restrictions. Refer to Air Cleaner Element Replacement and replace as necessary.

Fuel System
    • Inspect 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.
    • Inspect for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect the fuel injectors. Refer to Fuel Injector Solenoid Coil Test .
    • Inspect for 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 EVAP canister purge condition
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - Leaking fuel injectors--Refer to Fuel System Diagnosis .
       - An inaccurate mass air flow (MAF) sensor
       - Blockage on the inlet screen of the MAF sensor--Refer to Manifold Absolute Pressure Sensor Replacement .
       - Vacuum hoses that are split, kinked, or improperly connected
       - An air intake duct that is collapsed or restricted--Refer Intake Air Resonator Replacement .
       - An air filter that is dirty or restricted--Refer to Air Cleaner Element Replacement .
    • Inspect for the following conditions that may cause the engine to run lean:

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

       - Water intrusion in the HO2S connector
       - An exhaust leak between the HO2S and the engine--Refer to Exhaust Leakage .
       - Vacuum leaks
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - Restricted fuel injectors--Refer to Fuel System Diagnosis .
       - An inaccurate MAF sensor
       - Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
       - Vacuum hoses that are split, kinked, or improperly connected

Sensor/System

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
    • Verify that both fuel injector fuses are not open. An open fuel injector fuse causes four ignition coils and four fuel injectors not to operate. Inspect the ignition coil circuit and the injector circuits for an intermittent short to ground. Refer to Circuit Testing . Replace the fuse.
    • Soak the secondary ignition system with water from a spray bottle. Soaking the secondary ignition system may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as water is applied.
    • Inspect for proper ignition voltage output with the J 26792 Spark Tester.
    • Remove the spark plugs and inspect for the following conditions:
       - Correct heat range
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
       - Refer to Spark Plug Inspection .
    • 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.
    • Monitor the Misfire Current Counters while driving the vehicle within the conditions that the misfire occurred. If a misfiring cylinder can be located with a misfire, use the DTC P0300 table for diagnosis. Refer to DTC P0300 .
    • Inspect for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .

Engine Mechanical
    • Verify that the engine coolant temperature (ECT) 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 disable the fuel injectors 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 Overheating for diagnosis.
    • Inspect for excessive oil in the combustion chambers and leaking valve seals.
    • Test for low cylinder compression.
    • Inspect for incorrect basic engine parts, including the following:
       - The camshaft
       - The cylinder head
       - The pistons, etc.

Additional Inspections
    • Inspect the exhaust system for possible restrictions. Perform the following:
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the mufflers for heat distress or internal failure.
       - Inspect for plugged three-way catalytic converters by comparing the exhaust system back pressure on each side of the engine. Test back pressure by removing the secondary air injection (AIR) check valves near the exhaust manifolds. Refer to Restricted Exhaust .
    • Inspect the transmission torque converter clutch (TCC) for proper operation. Refer to Diagnostic Starting Point - Automatic Transmission .