GM Service Manual Online
For 1990-2009 cars only

Inspection/Tests

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 position.

Preliminary

    â€¢ Refer to Important Preliminary Inspections Before Starting in Symptoms - Engine Controls .
    â€¢ Search for bulletins.
    â€¢ Inspect the engine control module (ECM) grounds for being clean, tight, and in the proper locations. Refer to Power and Grounding Component Views in Wiring Systems and Engine Controls Schematics .
    â€¢ Verify the driver understands the operation of the transmission torque converter clutch (TCC) and A/C compressor operation as explained in the owners manual.

Sensor/System

Notice: Refer to Silicon Contamination of Heated Oxygen Sensors Notice in the Preface section.

    â€¢ Inspect for an engine coolant temperature (ECT) sensor that has shifted in value . Refer to Temperature Versus Resistance .
    â€¢ Inspect for proper operation of the manifold absolute pressure (MAP) sensor. Refer to DTC P0105 .

Fuel System

    â€¢ Test for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    â€¢ Inspect for a restricted fuel filter. Refer to Fuel System Diagnosis .
    â€¢ Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    â€¢ Verify that each fuel injector harness is connected to the correct fuel injector or cylinder. Relocate the fuel injector harnesses as necessary.
    â€¢ 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
       - Purging of a saturated EVAP canister
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - A leaking fuel pressure regulator--Refer to Fuel System Diagnosis .
       - Leaking fuel injectors--Refer to Fuel System Diagnosis .
       - MAP sensor operation--Refer to DTC P0105 .
       - Vacuum hoses that are split, kinked, or improperly connected
       - An air intake duct that is collapsed or restricted
       - An air filter that is dirty or restricted--Refer to Air Cleaner Element Replacement .
    â€¢ 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 in Engine Exhaust.
       - Vacuum leaks
       - Proper MAP sensor operation--Refer to DTC P0105 .
       - Incorrect fuel pressure--Refer to Fuel System Diagnosis .
       - Restricted fuel injectors--Refer to Fuel Injector Balance Test with Special Tool and Fuel Injector Balance Test with Tech 2 .
       - Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
       - Vacuum hoses that are split, kinked, or improperly connected

Ignition System

In order to properly inspect the spark plugs or to correct a condition, refer to the following procedures:

    â€¢  Spark Plug Inspection
    â€¢  Spark Plug Replacement .
    â€¢ Inspect for proper secondary ignition voltage output with the J 26792 (SA9199Z) Spark Tester.
    â€¢ An improper spark plug gap will cause a driveability condition. Gap the spark plugs using a wire gage gap tool.
    â€¢ If the spark plugs are found to be fouled, determine the cause before replacing the spark plugs.
    â€¢ Inspect for damaged or misaligned spark plug boots.
    â€¢ Inspect the electronic ignition (EI ) module for a proper ground connection.
    â€¢ Monitor the Misfire Current Counters while driving the vehicle in the conditions that the symptom occurred. If a misfiring cylinder can be located, use the DTC P0300 table for diagnosis. Refer to DTC P0300 .

Additional Inspections

    â€¢ Visually and physically inspect vacuum hoses for splits, kinks, and proper connections and routing as shown on the Vehicle Emission Control Information label.
    â€¢ Inspect the transmission torque converter clutch (TCC) for proper operation. Refer to Torque Converter Diagnosis in Automatic Transmission-VT25-E or Torque Converter Diagnosis in Automatic Transaxle-AF33-5.
    â€¢ Test the A/C clutch for proper operation. Refer to Symptoms - HVAC Systems - Manual in HVAC Systems.
    â€¢ Inspect the exhaust system for possible restrictions. Refer to Restricted Exhaust in Engine Exhaust.