Modes of Operation

MODES OF OPERATION
The Powertrain Control Module (PCM) monitors information from several sensors to determine fuel requirements based on engine operating conditions. The fuel is delivered under one of several conditions called modes. All modes are controlled by the PCM as described below.

Starting Mode
When the ignition is first turned "ON" (before engaging starter), the PCM energizes the fuel pump relay for a calibrated time (approximately 2 seconds) to allow the fuel pump to build up pressure. The PCM also checks the Engine Coolant Temperature (ECT) and Throttle Position (TP) sensors to determine the proper air/fuel ratio for starting. This ranges from approximately 1.55:1 at -40°C (-40°F) to 14.7:1 at 110°C (230°F) engine coolant temperature. The PCM controls the amount of fuel delivered by changing how long the injectors are energized. This is done by pulsing the injectors for very short times.

Clear Flood Mode
If the engine floods, it can be cleared by pushing the accelerator pedal down all the way while cranking the engine. The PCM then de-energizes the fuel injectors and holds them de-energized as long as the throttle stays wide open and the engine speed is below 600 RPM. If the throttle position becomes less than 70%, the PCM returns to the starting mode.

Run Mode
The run mode consists of both "Open Loop" or "Closed Loop" operation.
When the engine is first started and engine speed is above 400 RPM, the system goes into "Open Loop" operation. In "Open Loop," the PCM ignores the signal from the Oxygen Sensor (O2S) and calculates the air/fuel ratio based on inputs from the ECT and Manifold Absolute Pressure (MAP) sensors.

The system stays in "Open Loop" until the following conditions are met:

1. The Oxygen Sensor has varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature.)
2. The Engine Coolant Temperature (ECT) sensor is above a specified temperature.
3. A specific amount of time has elapsed after starting the engine.

he specific values for the above conditions vary with different engines and are stored in the Erasable Programmable Read Only Memory (EPROM) portion of the PCM. When these values are met, the system goes into "Closed Loop" operation.

In "Closed Loop," the PCM calculates the air/fuel ratio (injector "ON"-time) based on the signal from the O2S. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration Mode
When the driver depresses 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 amount of fuel required is based on throttle position, manifold air pressure and engine speed.

Fuel Cutoff Mode
To prevent possible engine damage from over-speed, the PCM cuts off fuel from the injectors when engine speed is above approximately 6280 RPM. On vehicles equipped with automatic transmission, fuel is cut off at 4000 RPM in park or neutral. The PCM also cuts off fuel to the injectors at vehicle speeds above 108 mph to prevent tire damage. Also see "Rapid Deceleration" under "Deceleration Mode."

Converter Protection Mode
The PCM constantly monitors engine operation and estimates conditions that could result in high converter temperatures. If the PCM determines the converter may overheat, it causes the system to return to "Open Loop" operation and enriches the fuel mixture.

Deceleration Mode
When the driver releases the accelerator pedal, air flow into the engine is reduced. The corresponding changes in throttle position and manifold air pressure are relayed to the PCM which reduces the injector pulse width to reduce fuel flow. If the deceleration is very rapid, or for long periods (such as long closed throttle coast-down), the PCM shuts "OFF" fuel completely.

Battery Voltage Correction Mode
When battery voltage is low, the PCM can compensate for the weak spark by:

^ Increasing the amount of fuel delivered.
^ Increasing the idle RPM.
^ Increasing ignition dwell time.