Fuel System Operating Modes

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

• Starting
  
  With ignition switch in the ON position, before engaging the starter, the PCM energizes the fuel pump relay for 2 seconds allowing the fuel pump to build up pressure. The PCM first tests speed density, then switches to the MAF sensor. PCM also uses the ECT, TP and MAP sensors to determine the proper air/fuel ratio for starting. PCM controls amount of fuel delivered in the starting mode by changing the pulse width of the injectors. This is done by pulsing the injectors for very short times.
• Clear Flood
  
  If the engine floods, clear engine by pressing the accelerator pedal down to the floor and then crank engine. When TP sensor is at Wide-Open Throttle (WOT), the PCM reduces the injector pulse width in order to increase the air/fuel ratio. The PCM 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 PCM returns to the 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 PCM ignores the signal from the HO2S and calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP and TP sensors. The system stays in Open Loop until both HO2S have varying voltage output, showing that they are hot enough to operate properly (his depends upon engine temperature), ECT sensor is above a specified temperature. and a specific amount of time has elapsed after starting the engine. Specific values for the mentioned 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 PCM calculates the air/fuel ratio (injector on-time) based upon the signal from various sensors, but mainly from the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.
• Acceleration
  
  When the driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. To prevent possible hesitation, PCM increases the pulse width to the injectors to provide extra fuel during acceleration. PCM determines the amount of fuel required based upon the throttle position, coolant temperature, manifold air pressure, mass airflow, and engine speed.
• Deceleration
  
  When the driver releases the accelerator pedal, air flow into the engine is reduced. The PCM reads the corresponding changes in throttle position, manifold air pressure, and mass airflow. The PCM 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 protect 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).