GM > Engine > Fuel Control > Fuel Cutoff, DFCO


RPM Limits
The VCM has a number of RPM limits that disable fuel if exceeded. The Cutoff RPM is the RPM at which fuel is disabled, spark is cut or the ETC begins limiting and the Resume RPM is the RPM at fuel is re-enabled.


Deceleration Fuel Cutoff (DFCO)
DFCO can be broken down into 5 main modes of operation.

  1. Qualifications to consider fuel cutoff (Enable)
  2. Spark threshold/ ramp down values and enable Qualification hysteresis values while getting to fuel cut point. (Entering)
  3. Spark values while in fuel cutoff (In)
  4. Qualifications to stay in fuel cutoff (Exit)
  5. Spark ramp up values to exit fuel cutoff (Exiting)
On later vehicles, DFCO will progressively cut fuel to each cylinder rather than the simpler on/off used on earlier vehicles.


DFCO Qualification
DFCO must pass a series of enable tests to even be considered. If any of the tests are not passed, the DFCO process will not be started. (Fuel is still on)

Fuel Tab - DFCO Enable Normal Section:


Fuel Tab - DFCO Enable Clutch Transition Section: (Not used on all models)
These values over ride ANY of the DFCO Enable Normal Section qualifications (Enable, Entering, Exit) if recent clutch activity has been detected.

These values will remain active for a short time period before reverting back to the normal qualifications.


DFCO Entry:
Enable tests passed, VCM selects spark threshold/ ramp down values and Enable Qualification hysteresis values while getting to fuel cut. (DFCO is considered active and Fuel is still on)

Spark Tab - DFCO Entry, Active, Exit Spark Section:


Fuel Tab - DFCO Enable Normal Section:


In DFCO:
All of the qualifications have been met, Spark Cut Threshold has been reached and the fuel has been turned off. VCM sets the spark advance and looks for a reason to exit DFCO.

Spark Tab - DFCO Entry, Active, Exit Spark Section:


Fuel Tab - DFCO Enable Normal Section:


Exiting DFCO:
VCM has found a reason to turn the fuel back on and start the spark exiting process.

Spark Tab - DFCO Entry, Active, Exit Spark Section: