GM > Engine > Fuel Control > General

Stoich Air Fuel Ratio (Stoich AFR)

• Stoich AFR: This value is the Stoichiometric AFR value. This is the reference AFR used by all fuel calculations inside the VCM. Deviations from this value are represented by divisors where greater than one is richer and less than one is leaner. Changing this value will effect all fuel calculations.
• Stoich AFR Table: This table is used for vehicles with the dual fuel option (alcohol)
  The base stoichiometric fuel/air rate is determined by a returned value from this table. The position on the table is determined by the amount of alcohol in the fuel system as reported by the vehicles fuel composition sensor. The rate ranges from 0% to 100% alcohol.

Afterstart Enrichment
The afterstart enrichment function provides the required enrichment after engine cranking and then a gradual ramp (decay) into stoichiometric fuel delivery and closed loop operation. The afterstart enrichment adds to the current fuel/air multiplier (equivalence ratio). If the engine enters closed loop then afterstart enrichment is zeroed.

• Initial Adder vs. ECT: Initial adder versus engine coolant temperature.
• Initial Adder vs. IAT: Initial adder versus inlet air temperature.
• Decay Delay: Initial Adder is held for this long before decay begins.
• Decay Step: Time delay between successive decays of the afterstart enrichment value once initial delay is complete.
• Decay Mult: Multiplier applied to current afterstart enrichment value every decay step.
• Initial Adder Soaktime Mult: Multiplier to Initial Adder vs. ECT value based on soaktime (time between engine off and engine restart).
• Hotsoak Enrichment: Hotsoak initial extra enrichment adder based on ECT and soaktime (time between engine off and engine restart).

PN/Drive Enrichment
During openloop operation, extra fuel is added during a transition from PN to Drive for A4 transmissions.

• Initial Adder vs. ECT: Initial adder versus engine coolant temperature.
• Decay Delay: Initial Adder is held for this long before decay begins.
• Decay Step: Time delay between successive decays of the PN-Drive enrichment value once initial delay is complete.
• Decay Mult: Multiplier applied to current afterstart enrichment value every decay step.

Startup AFR Correction (V6 Engines)
Startup AFR correction is the calculation of the factor that is subtracted from the current commanded AFR during startup/warmup. It decays to zero depending on the decay factor and is disabled once the engine reaches closed loop operation.

• Enable Time vs ECT: These tables control the delay before Startup AFR Correction is enabled.
• Initial Value vs Startup ECT: This is the initial value of the AFR correction when the engine is started. It decays to zero based on the decay factor below.
• Decay vs Startup ECT vs. RPM: This is the decay rate of the Initial AFR correction value. Higher numbers mean a faster decay.

Injector Control

• Injector Offset Select: Selects either MAP or Vacuum based injector offset table in use.
• Injector Offset vs. Battery Voltage vs. KPA Vac: This value is added to the injector pulse width as the delay time it takes for the injector to operate. It varies with battery voltage and manifold vacuum.
• Injector Offset vs. Battery Voltage vs. KPA MAP: This value is added to the injector pulse width as the delay time it takes for the injector to operate. It varies with battery voltage and manifold pressure.

  
  

• Injector Flow Rate vs. KPA Vac: This value is the Injector Flow rate in g/sec. It is used to tell the VCM the size of the injectors and also how flow rate varies with manifold vacuum (ie. varying pressure characteristics).
• Injector Flow Rate vs. KPA MAP: This value is the Injector Flow rate in g/sec. It is used to tell the VCM the size of the injectors and also how flow rate varies with manifold pressure (ie. varying pressure characteristics). On some vehicles this table references absolute pressure drop across the injectors (hence it includes the fuel rail pressure).

  
  

• Injector Constant: This value is used to describe the injector flow rate in liters per gram per second. It includes a scaling factor to account for the engine displacement. Higher values for smaller injectors, smaller values for larger injectors.
• Injector Pulse Width Multiplier vs. VAC: This value multiplies the calculated injector pulse width in relation to manifold vacuum.

  
  

• Fuel Pressure Modifier vs. Battery Voltage: This table defines how fuel pressure varies with battery voltage.

  
  

• Skew vs. Cylinder (vehicle dependent): Many V6 and L4 calibrations allow for injector skewing to tune fueling variations between cylinders. The skew factor is a static multiplier of the current calculated injector pulse width for that cylinder. A value of 1.0 provides no skew.
• Short Pulse Limit: Injector Pulses shorter than this will be increased by the amount specified in Short Pulse Adder.
• Short Pulse Adder: Adder to pulses shorter than Short Pulse Limit.
• Minimum Injector PW: Minimum allowed Injector Pulse Width (after Short Pulse adder has been added if necessary).
• Default Injector PW: Default Pulse width used when Injector Pulse Width is less than Minimum Injector PW.

General Glow Plugs(Diesel Engines)

• Glow Plug Timer - Start: The amount of time the glow plugs will be on when the ignition is on but the engine is not running.
• Glow Plug Timer Start 2: The amount of time the glow plugs will be on when the ignition is on but the engine is not running.
• Glow Plug Timer - Run: The amount of time the glow plugs will be on when the engine is running.

Cranking Fuel
These tables control injector priming pulses and cranking fuel delivery.

• Cranking FA Mult Stage 1 & 2 vs. Cranking Time: These tables are FA multipliers that determine fuel delivery in relation to time. As cranking continues the AFR is selected from lower rows of the table until it finally stays at the last row in the Stage 2 table. Usually you will see a pulsing fuel pattern in these tables.
• Cranking Leanout Factor vs. ECT vs. RPM: This table leans out the cranking fuel based in relation to ECT and RPM. It is a value between 0 and 1 and multiplies the Fuel/Air Mult Stage 1 & Stage 2 values.

  
  

• Prime Fuel Disable RPM: Above this RPM, 1st and 2nd prime pulses will not be delivered.
• Soaktime - Runtime Limit: Enable Soaktimer modifiers if last engine run time was less than this value and Soaktime ECT condition is met.
• Soaktime ECT Threshold: Enable Soaktimer modifiers if engine coolant temp is less than this value and Soaktime Runtime Limit condition is met.
• Prime Re-enable Time: Prime Pulse is delivered if the previous Prime Pulse was delivered more than this time ago.
• BARO Gain: Modifier for barometric conditions.
• Prime Runtime: Engine runtime allowed to deliver priming pulses.
• Prime Pulse Mass: Mass of fuel delivered thru all injectors when key is switched on and fuel pressure delay has expired.
• First Pulse Mass: Mass of fuel delivered thru all injectors when 1st Pulse Delay has expired and fuel pressure delay has expired.
• First Pulse Delay: Delay before 1st Pulse Mass is delivered.
• Second Pulse Mass: Mass of fuel delivered thru all injectors when 1st and 2nd Pulse Delay has expired and fuel pressure delay has expired.
• Second Pulse Delay: Delay after 1st pulse before 2nd Pulse Mass is delivered.
• Fuel Pressure Delay: Delay after key on, before Prime Pulse is delivered to allow for fuel pump to build pressure.

  
  

• Flood Clear Enable %TPS: If the Enable TPS value is exceeded then fuel is cut during cranking.
• Flood Clear Disable %TPS: Fuel is resumed when the disable TPS is reached.

Engine Over Temperature

• Engine Over Temp Fuel Mult vs. RPM: Fuel/Air multiplier used when engine over temperature protection is enabled.