Fuel System Operating Modes

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

• Starting

  During engine starts, PCM/VCM delivers one injector pulse for each distributor reference pulse received (synchronized mode). Injector pulse width is based upon coolant temperature and throttle position. PCM/VCM determines air/fuel ratio when throttle position is less than 80 percent open. Engine starting air/fuel ratio ranges from 0.8:1 at -40°F (-40°C) to 16.8:1 at 230°F (110°C). At lower coolant temperatures, injector pulse width is wider (richer air/fuel mixture ratio). When coolant temperature is high, injector pulse width becomes narrower (leaner air/fuel ratio).

• Clear Flood

  If engine is flooded, driver must depress accelerator pedal to Wide Open Throttle (WOT) position. At this position, PCM/VCM adjusts injector pulse width equal to an air/fuel ratio of 16.5:1. This air/fuel ratio is maintained as long as throttle remains in wide open position and engine speed is less than 600 RPM. If throttle position becomes less than 65 percent open and/or engine speed exceeds 600 RPM, PCM/VCM changes injector pulse width to that used during engine starting (based upon coolant temperature and manifold vacuum).

• Heavy Acceleration

  PCM/VCM provides fuel enrichment during heavy acceleration. Sudden opening of throttle valve causes rapid increase in MAP or MAF signal. Pulse width is directly related to MAP or MAF, throttle position and coolant temperature. Higher MAP or MAF and wider throttle angles give wider injector pulse width (richer mixture). During enrichment, injector pulses are not in proportion to distributor reference signals (non-synchronized). Any reduction in throttle angle cancels fuel enrichment.

• Deceleration

  During normal deceleration, fuel output is reduced. This reduction in available fuel serves to remove residual fuel from intake manifold. During sudden deceleration, when MAP or MAF, throttle position and engine speed are reduced to preset levels, fuel flow is cut off completely. This deceleration fuel cut-off overrides normal deceleration mode. During either deceleration mode, injector pulses are not in proportion to distributor reference signals.

• Battery Voltage Correction

  PCM/VCM compensates for low battery voltage by increasing injector pulse width and increasing idle RPM. PCM/VCM 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/VCM, 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).