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Elevated Idle Description for LLY Duramax

This system comes factory programmed to the 6.6L LLY Duramax (8th digit of VIN is “2”)

Manual High Idle System Description

The diesel engine has a high idle system to improve the warm-up time of the engine in cold weather conditions. This system allows the engine control module (ECM) to increase the idle speed above the normal calibrated value.

The ECM increases the idle speed using the following adjustments:

  • The fuel injection timing is changed.
  • The fuel injection quantity is changed.
  • The turbocharger vane position is commanded closed. the vane position will be farther closed than any other normal operating condition.

The instrument panel will indicate the high idle system is active one of two ways:

  • The Driver Information Center (DIC) will indicate an active high idle system on Light Duty Trucks.
  • An indicator lamp will flash on medium duty trucks.

High Idle Speed Enable and Disable

To enable or disable the high idle system perform the following procedure:

  • Turn the ignition ON with the engine OFF.
  • Depress the accelerator pedal to the floor and hold down.
  • While the accelerator pedal is depressed, depress the brake pedal 3 times in less than 8 seconds.
  • Release the accelerator pedal.
  • Start the engine.

When the procedure is followed the engine idle speed will slowly increased to the calibrated high idle speed. This is 1200 RPM for Light duty, and 1500 RPM for medium duty trucks.

The idle speed will return to normal if any of the following conditions occur:

  • There is brake, clutch, or throttle input from the driver.
  • The automatic transmission is shifted out of Park or Neutral.
  • The air temperature is more than 0 °C (32 °F).
  • The engine coolant temperature (ECT) is more than 68 °C (154 °F) The vehicle speed exceeds 0 MPH (0 km/h).

The high idle system will reactivate automatically when the following conditions occur:

  • The engine has been idling for more than 30 seconds.
  • The transmission is placed in Park or Neutral.
  • The vehicle speed is 0 MPH (0 km/h).
  • The ambient air temperature is less than 0 °C (32 °F).
  • The ECT is less than 68 °C (154 °F)
  • The brake, clutch and throttle pedals are not depressed.