Fuel Delivery Modes

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. All modes are controlled by the PCM and are described below.

Starting Mode
When the ignition switch is turned to the "ON" position (before engaging starter), the PCM energizes the fuel pump relay for two seconds allowing the fuel pump to build up pressure. The PCM first checks speed density, then switches to the Mass Air Flow (MAF) sensor. Engine Coolant Temperature (ECT), Throttle Position (TP), and Manifold Absolute Pressure (MAP) sensors are also used 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 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. The PCM then pulses the injectors at an air/fuel ratio of 20:1. The PCM holds 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%, the PCM returns to the starting mode.

RUN MODES
The run mode has two conditions called "Open Loop" and "Closed Loop."

Open Loop
When the engine is first started, and engine speed is above 400 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:

1. Both HO2S have varying voltage output, showing that they are hot enough to operate properly (This depends on temperature.).
2. The ECT sensor is above a specified temperature.
3. 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). When these values are reached, the system begins "Closed Loop" operation.

Closed Loop
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 amount of fuel required is 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 corresponding changes in throttle position, manifold air pressure, and mass air flow 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 to protect the warm up three-way catalytic converters.

BATTERY VOLTAGE CORRECTION MODE
When battery voltage is low, the PCM compensates for the weak spark delivered by the distributor by:

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

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 5500 RPM. To prevent tire damage on vehicles equipped with base tires, the PCM also cuts off fuel from the injectors at vehicle speeds above 108 mph.