GM Service Manual Online
For 1990-2009 cars only

Hesitation, Sag, Stumble

Checks

Action

DEFINITION: A momentary lack of response when the accelerator is pushed down. The condition can occur at any vehicle speed. The condition is usually more pronounced when the driver first attempts to make the vehicle move, such as acceleration from a stop sign. The condition may cause the engine to stall if severe enough.

Preliminary

    • Refer to Symptoms - Engine Controls .
    • Search for service bulletins.

Sensor/System

Check for the following conditions:

    • Proper operation of the manifold absolute pressure (MAP) sensor
    • Proper operation of the throttle position (TP) sensor
    • Proper operation of the engine coolant temperature (ECT) sensor
    • Check the oxygen sensor (O2S 1). The O2S 1 should quickly respond to the different throttle positions. If it does not, check the O2S 1 for silicon, or for other contaminants from the fuel or from the use of an improper RTV sealant. The sensor may have a white powdery coating. Silicon contamination causes a high but false O2S 1 signal voltage, indicating a rich exhaust indication. The PCM will reduce the amount of fuel delivered to the engine, causing a severe driveability problem.

Fuel System

Check for the following conditions:

    • The fuel pressure--Refer to Fuel System Diagnosis .
    • Contaminated fuel--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • The fuel injectors--Refer to Fuel Injector Balance Test with Tech 2 .
    • Conditions which can cause an engine to run rich, indicating that the long-term fuel trim is significantly in the negative range. Refer to Diagnostic Aids for DTC P0172 .
    • Conditions that can cause an engine to run lean, indicating that the long-term fuel trim is significantly in the positive range. Refer to Diagnostic Aids for DTC P0171 .
    • Proper operation of the evaporative emission (EVAP) control system

Ignition System

Check for the following conditions:

    • Proper ignition voltage output--Use the J 36012-A Spark Plug Jumper Wires and use the J 26792 Spark Tester, or equivalent.
    • Check the spark plugs for the following conditions:
       - Wet or fuel-fouled spark plugs
       - Cracks on the spark plugs
       - Wear
       - Improper spark plug gap
       - Burned electrodes or heavy deposits
       - Heavy deposits
    • If the spark plugs are fuel or oil-fouled, determine the cause of the fouling before you replace the spark plugs.
    • The ignition control module (ICM) for proper ground connection
    • The ignition coils for cracks or for carbon tracking

Additional Checks

Check for the following conditions:

    • Proper operation of the automatic transaxle. Refer to Road Test in Automatic Transmission 4T40-E/4T45-E.
    • The generator output voltage--Repair the generator if the output voltage is less than 11 volts, or more than 16 volts.
    • If you have completed all of the procedures, and if you have found no malfunctions, review the following items:
       - Inspect all electrical connections within the suspected circuit and/or systems.
       - Check the scan tool data. Refer to Scan Tool Data List .