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.

• P/N Fuel Cutoff RPM: If the current selected gear is Park or Neutral then this RPM limit is used.
• P/N Fuel Resume RPM: If the current selected gear is Park or Neutral then this RPM limit is used.
• P/N Fuel Resume RPM: If the current selected gear is Park or Neutral then this RPM limit is used.

  
  

• Bad VSS Fuel Cutoff RPM: If the Vehicle Speed Sensor is bad then this RPM limit is used.
• Bad VSS Fuel Resume RPM: If the Vehicle Speed Sensor is bad then this RPM limit is used.

  
  

• Cutoff RPM Hysteresis: The amount the RPM must drop to disable the RPM Limiter.
• RPM Hysteresis (Gear): The amount the RPM must drop to disable the RPM Limiter in Gear.
• RPM Hysteresis (P/N): The amount the RPM must drop to disable the RPM Limiter in Park/Neutral.
• Fuel Cutoff RPM (P/N) vs. ECT: The RPM Limit when in Park or Neutral.
• Fuel Cutoff RPM (In Gear) Modifier vs. ECT: The RPM Limit when in gear is multiplied by this value (cold engine reduction).

  
  

• Extreme Cutoff RPM: Fuel cut will enable regardless of other settings if RPM exceeds this value.
• Extreme Resume RPM: If extreme cutoff is active, RPM must drop below this value to resume fuel.

  
  

• Fuel Cutoff RPM vs. Gear: If the current selected gear is not Park or Neutral then this table is used to determine the current RPM limit.
• Fuel Cutoff Delay vs. Startup ECT: This table determines how long the VCM uses the Fuel Cutoff RPM vs. Startup ECT table for the RPM limit test. After this delay has expired (varies with engine temp) the VCM uses the other RPM limits.
• Fuel Cutoff RPM vs. Startup ECT: This table is used for the RPM limit whilst the Engine Run Time is less than the Fuel Cutoff Delay above.
• Fuel Cutoff RPM - 1st Gear Reduce: When in first gear the RPM limiter can be reduced over time by using this table.

  
  

• Fuel Cutoff RPM Reduction vs. ERT vs ECT: Startup RPM limit reduction in relation to engine run time and ECT.

  
  

• RPM Limit - Spark: RPM Limit will use Spark if enabled.
• RPM Limit - Fuel Cut: RPM Limit will use Fuel Cut if enabled.
• RPM Limit - ETC: RPM Limit will use Electronic Throttle Control if enabled.
• RPM Limit - Method Delay: RPM Limit will switch from spark/fuel cut to ETC after this time.

  
  

• RPM Limit - Accel Coeff: Coefficient used to filter engine acceleration, lower numbers mean more filtering.
• RPM Limit - Accel Thresh: RPM below the RPM Limit to begin Acceleration based RPM Limiting.
• RPM Limit - Accel Max: Above this engine acceleration rate, acceleration based RPM limiting will begin.

  
  

• RPM Limit Torque vs. RPM Delta vs. Gear: This is the commanded torque value based on the RPM error that controls the RPM limiter stability. Higher numbers mean faster acting limiter but more oscillation (instability) around the limiter RPM.
• RPM Limit Lamp RPM: Engine Overspeed lamp will light above this RPM (if fitted).

  
  

• ETC Limit Enable: This switch enables/disables Electronic Throttle Control engine speed limiting functions.
• ETC Fuel Cutoff RPM vs. Gear: This table is used if an electronic throttle is fitted. It is an RPM limit that closes the throttle if exceeded.

  
  

• Fuel Cutoff RPM: The RPM that fuel will be cut.
• Fuel Resume RPM: The RPM that when fuel will resume.
• Fuel Resume RPM Excessive: If excessive RPM limiter activity is detected then resume limits will switch to these values.

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)

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:

• Temp: Engine Coolant temperature must exceed this value.
• Enable Speed vs. Baro: The vehicle speed must exceed this value.
• RPM vs. Gear Entry: Engine RPM must exceed table value.
• TPS vs. RPM: Throttle position must be below this table value.
• Map vs. Baro Entry: Map must be below this table value.

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.

• Temp: Engine Coolant temperature must exceed this value.
• Speed: The vehicle speed must exceed this value.
• RPM: Engine RPM must exceed table value.
• TPS: Alternate VCM %Throttle position Change must be below this table value.
• Map: Map must be below this table value.
• TPS Hyst: Adds to Alternate VCM %Throttle position Change value.

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:

• Entry Threshold vs. RPM: This is the degree spark advance that must be reached to turn the fuel off.
• Entry Blending vs. RPM: This is the spark ramp out rate to reach the Entry Threshold vs. RPM spark value.
  Spark is removed at a rate of 12.5msec at the Entry Blending multiplier value. If your current spark advance is 20 degrees @3000 rpm, your Entry threshold is 5 degrees, and your removal rate is 0.010, the VCM will remove .2 degrees (20 x 0.010) of spark every 12.5 milliseconds until the Entry Threshold of 5 degrees is reached.

• Speed Hyst: Subtracts from Enable Speed vs. Baro table value.
• RPM Hyst: Subtracts from RPM vs. Gear Entry table value.
• TPS: Adds to TPS vs. RPM table value.
• Map Hyst: Adds to Map vs. Baro table value.

  
  

• Enable Temp: ECT must be above this to enable DFCO.
• Disable Temp: If ECT drops below this while DFCO is enabled, then DFCO will disable.
• Enable RPM: RPM must be above this to enable DFCO.
• Disable RPM: If RPM drops below this while DFCO is enabled, then DFCO will disable.
• Enable Speed: Speed must be above this to enable DFCO.
• Disable Speed: If Speed drops below this while DFCO is enabled, then DFCO will disable.
• Enable Cylair: Cylair must be below this to enable DFCO.
• Disable Cylair: If Cylair goes above this while DFCO is enabled, then DFCO will disable.
• Enable TPS: TPS must be below this to enable DFCO.
• Disable TPS: If TPS goes above this while DFCO is enabled, then DFCO will disable.

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:

• Active Advance vs. RPM A: This is the degree of spark advance while the fuel is turned off.
• Min TPS for Table A: TPS must exceed this value to use table A.
• Active Advance vs. RPM B: This is the degree of spark advance while the fuel is turned off.
• Max TPS for Table B: TPS must be under this value to use table B.

  
  

• DFCO Entry Spark: Spark will ramp down to this value before DFCO enables.

• Enable Speed vs. Baro: The vehicle speed must exceed this value.
• RPM vs. Gear Exit: Engine RPM must exceed table value.
• TPS vs. RPM: Throttle position must be below this table value.
• Map vs. Baro Exit: Map must be below this table value.

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:

• Exit Blending vs. TPS: Spark is added at a rate of 12.5msec at the Exit Blending value. If your current spark advance (Main spark table, not Active Advance vs. RPM A/B) is 20 degrees @3000 rpm, and your add rate is 0.286, the VCM will add .286 degrees of spark every 12.5 milliseconds until the main spark table value is reached.
• Exit Blending CT (Clutch Transition): This value over rides the Exit Blending vs. TPS table if clutch activity has been detected.
• Stall RPM: This is the lower RPM limit for DFCO fuel cut.

  
  

• DFCO Exit Spark: Spark will ramp up from this value when exiting DFCO.