Fuel System Operating Modes

Internal PCM calibration controls fuel delivery during starting, clear flood mode, deceleration and heavy acceleration.

• Starting
  
  When ignition is first turned ON, the control module energizes the fuel pump relay for 2 seconds. This allows fuel pump to build pressure in the fuel system. The control module calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP and TP sensors. The system stays in starting mode until engine speed reaches a predetermined RPM.
• Clear Flood
  
  If engine floods, clear engine by pressing the accelerator pedal down to floor and then crank engine. When TP sensor is at Wide Open Throttle (WOT), the control module reduces the fuel injector pulse width in order to increase the air to fuel ratio. The control module holds this injector rate as long as the throttle stays wide open and engine speed is below a predetermined RPM. If throttle is not held wide open, the control module returns to starting mode.
• Run Mode
  
  The run mode has 2 conditions called open loop and closed loop. When engine is first started and engine speed is above a predetermined RPM, the system begins open loop operation. The control module ignores the signal from the HO2S. The control module calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP, and TP sensors. The system stays in open loop until meeting the following conditions: The HO2S has varying voltage output, showing that the HO2S is hot enough to operate properly, ECT sensor is above a specified temperature and specific amount of time has elapsed after starting engine.

  Specific values for the conditions exist for each different engine and are stored in the Electrically Erasable Programmable Read-Only Memory (EEPROM). The system begins closed loop operation after reaching these values. In closed loop, the control module calculates the air/fuel ratio, injector ON time, based upon the signal from various sensors, but mainly from the HO2S. This allows air/fuel ratio to stay very close to 14.7:1.

• Acceleration
  
  When driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly. To prevent possible hesitation, the control module increases the pulse width to the injectors to provide extra fuel during acceleration. This is also known as power enrichment. The control module determines the amount of fuel required based upon TP, ECT, MAP, MAF and engine speed.
• Deceleration
  
  When driver releases the accelerator pedal, air flow into the engine is reduced. The control module monitors the corresponding changes in the TP, MAP and MAF. The control module shuts OFF fuel completely if the deceleration is very rapid or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to prevent damage to the catalytic converters.
• Battery Voltage Correction
  
  PCM compensates for low battery voltage by increasing injector pulse width and increasing idle RPM. PCM is able to perform these commands because of a built-in memory/learning function.
• Fuel Cut-Off
  
  When ignition is turned off, injectors are de-energized to prevent dieseling. Injectors are not energized if RPM reference pulses are not received by the PCM, even with ignition on. This prevents flooding before starting. Fuel cut-off also occurs at high engine RPM or excessive vehicle speed to prevent internal damage to engine. Some models may also cut off fuel injector signals during periods of sudden, closed throttle deceleration (when fuel is not needed).
• Fuel Trim
  
  The control module controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The control module monitors the HO2S signal voltage while in closed loop and regulates the fuel delivery by adjusting the pulse width of the injectors based on this signal. The ideal fuel trim values are around zero percent for both short and long term fuel trim. A positive fuel trim value indicates the control module is adding fuel in order to compensate for a lean condition by increasing the pulse width. A negative fuel trim value indicates that the control module is reducing the amount of fuel in order to compensate for a rich condition by decreasing the pulse width. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltage. These changes fine tune the engine fueling. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. A scan tool can be used to monitor the short and long term fuel trim values. The long term fuel trim diagnostic is based on an average of several of the long term speed load learn cells. The control module selects the cells based on the engine speed and engine load. If the control module detects an excessively lean or rich condition, the control module will set a fuel trim DTC.