GM Service Manual Online
For 1990-2009 cars only

Checks

Action

DEFINITION: Engine power variation under steady throttle or cruise. Feels like the vehicle speeds up and slows down with no change in the accelerator pedal.

Preliminary

    • Refer to Important Preliminary Checks Before Starting in Symptoms - Engine Controls .
    • Search for bulletins.

Be sure that the driver understands the transmission torque converter clutch (TCC), and A/C compressor operation as explained in the owners manual. Inform the customer how the TCC and the A/C system operate.

Sensor/System

    • Check the heated oxygen sensors (HO2S)s. The HO2Ss should respond quickly to different throttle positions. If they do not, check the HO2S for the following conditions:
       - Contaminants from the fuel
       - Improper use of RTV
       - The sensors may have a white powdery coating. This will result in a high but false signal voltage (rich exhaust indicated). The powertrain control module (PCM) will reduce the amount of fuel delivered to the engine causing a severe driveability condition.
    • Check the mass air flow (MAF) sensor and intake air system for proper connections.
    • Check the exhaust gas recirculation (EGR) system for proper operation.

Fuel System

    • Check the fuel pressure. Refer to Fuel System Diagnosis .
    • Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Check for conditions which can cause an engine to run rich and/or lean.

Ignition System

    • Check for proper ignition voltage output using the J 26792 Spark Tester.
    • Check for damaged ignition coils within the ignition coil module assemblies.
    • Check for damaged or misaligned spark plug boots.
    • Check for a missing or damaged ignition module ground spring.
    • Check the spark plugs. Refer to Spark Plug Inspection .

Additional Checks

    • Inspect the vacuum hoses for splits, kinks, and improper connections or routing.
    • Check torque converter clutch (TCC) operation. Refer to Torque Converter Diagnosis in Automatic Transaxle.
    • Check for a restrictive exhaust system. Refer to Restricted Exhaust .