Modes of Operation
The powertrain control module (PCM) looks at voltages from several sensors to determine how much fuel to give the engine. The fuel is delivered under one of several conditions called modes. The PCM controls all modes
Starting Mode
With the ignition switch in the ON position (before you engage the starter), the PCM energizes the fuel pump relay for 2 seconds allowing the fuel pump to build up pressure. The PCM first checks speed density, then switches to the mass air flow (MAF) sensor. The PCM also uses the engine coolant temperature (ECT), throttle position sensor (TPS), and manifold absolute pressure (MAP) sensors to determine the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33°F) to 14.7:1 at 94°C (201°F) running temperature. The PCM controls the 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 Mode
If the engine floods, clear the engine by pushing the accelerator pedal down all the way. The PCM then pulses the injectors at an air/fuel ratio of 20:1. The PCM hold this injector rate as long as the throttle stays wide open and the engine speed is below 300 RPM. If the throttle position becomes less than 80 percent, the PCM returns to the starting mode.
Run Mode
The run mode has two conditions called Open Loop and Closed Loop. When the engine is first started, and the engine speed is above a predetermined RPM, the system begins the 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 sensor. The system stays in open loop until meeting the following conditions:
| • | Both HO2S have varying voltage output, showing that they are hot enough to operate properly. (This depends on the 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, 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 on the signal from various sensors, but mainly the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.
Acceleration Mode
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, the PCM increases the pulse width to the injectors to provide extra fuel during acceleration. The PCM determines the amount of fuel required based on throttle position, coolant temperature, manifold air pressure, mass air flow and engine speed.
Deceleration Mode
When the driver releases the accelerator pedal, air flow into the engine is reduced. The PCM looks at the corresponding changes in throttle position, manifold air pressure and mass air flow. 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 warm-up three-way catalytic converters.
Fuel Cutoff Mode
To prevent possible engine damage from over-speed, the PCM cuts off fuel from the injectors when the engine speed is above approximately 5,800 RPM. Refer to Deceleration Mode.
Battery Voltage Correction Mode
When battery voltage is low, the PCM compensates for the weak spark delivered by the distributor in the following ways:
| • | Increasing the amount of fuel delivered |
| • | Increasing the idle RPM |
| • | Increasing ignition dwell time |
