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 starter, the PCM energizes the fuel pump relay for 2seconds allowing fuel pump to build pressure. Speed density is determined by inputs from the engine RPM, the IAT and MAP sensors. 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. The PCM controls the amount of fuel delivered in the starting mode by changing the width of the fuel injector pulse.
• Clear Flood
  
  If the engine floods, clear the engine by pushing 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-to-fuel ration. The PCM maintains 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, PCM returns to the starting mode.
• Run Mode
  
  The run mode has 2conditions. These conditions are 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 the following conditions are met: Both HO2Ss have varying voltage output, showing that they are hot enough to operate properly. This depends upon engine temperature. The ECT sensor is above a specified temperature. A specific amount of time has elapsed after starting the engine. Specific values for the above conditions exist for each different engine. These values 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, fuel injector ON time, based upon the signal from various sensors, but mainly form 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, the air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. In order to prevent possible hesitation, the PCM increases the pulse width to the fuel injectors in order to provide extra fuel during acceleration. The PCM determines amount of fuel required based upon the throttle position, coolant temperature, MAP, MAF and engine speed.
• Deceleration
  
  When the driver releases the accelerator pedal, the air flow into the engine is reduced. The PCM reads the corresponding changes in the TP, MAP and MAF sensors. The PCM shuts OFF fuel completely if deceleration is very rapid or for long periods, such as during a long, closed-throttle coast-down. The fuel shuts OFF in order to protect the TWC.
• 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. Fuel injector signals are cut off during periods of sudden, closed throttle deceleration (when fuel is not needed).