GM Service Manual Online
For 1990-2009 cars only

Lack of Power, Sluggish or Spongy

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.
    • Remove and check the air filter element for dirt, or for being restricted. Replace as necessary.

Fuel System Checks

    • Check for low fuel pressure. Refer to Fuel System Pressure Test .
    • Check for a restricted fuel filter.
    • Check for contaminated fuel. Refer to Fuel Quality Diagnosis .
    • Check items which can cause an engine to run rich (long term fuel trim near -99%).
    • Check items that can cause an engine to run lean (long term fuel trim near 100%).

Sensor/System Checks

    • Using a scan tool, monitor KS system for excessive spark retard activity.
    • Check the EGR system for proper operation.

Ignition System

Remove 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. The spark plug gapping should be done using a wire gauge gap tool (J 41319). If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.

    • Check for loose ignition control module ground.
    • Spark plugs for proper heat range.
    • Check for proper ignition voltage output with spark tester J 26792. Refer to Secondary Ignition Diagnosis .

Engine Mechanical

Check for the following conditions:

    • Excessive oil in combustion chamber may indicate leaking valve seals. Refer to Oil Consumption Diagnosis .
    • Low cylinder compression. Refer to Engine Compression Test .
    • Incorrect basic engine parts. Inspect the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Refer to Base Engine Misfire Diagnosis .

Additional Checks

    • Check the exhaust system for possible restrictions. Refer to Restricted Exhaust System Check .
    • Check the Torque Converter Clutch (TCC) for proper operation. Refer to Flexplate/Torque Converter Vibration Test in Automatic Transmission -- 4T65-E.
    • Traction Control system brake switch assembly. Loss of performance can result from an improperly adjusted or a failed extended travel brake switch. The PCM operates in the Torque Management mode when the PCM detects the brake pedal and throttle are depressed at the same time.