Fuel Delivery System

PURPOSE
The function of the fuel metering system is to deliver the correct amount of fuel to the engine under all operating conditions. Fuel is delivered to the engine by individual fuel injectors mounted in the intake manifold near each cylinder.

OPERATION
The main control sensor is the Oxygen (02) sensor, which is located in the exhaust manifold. This sensor tells the Powertrain Control Module (PCM) how much oxygen is in the exhaust gas, and the PCM changes the air / fuel ratio to the engine by controlling the fuel injectors. The best mixture to minimize exhaust emissions is 14.7:1, which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air / fuel ratio, the fuel injection system is called a "Closed Loop" system.

COMPONENTS
The fuel metering system consists of the following components:

^ Fuel tank
^ Fuel pump
^ Fuel lines
^ Fuel pump electrical circuit,
^ Fuel injector rail
^ Fuel injectors
^ Pressure regulator

^ Throttle body assembly, including:

^ Idle Air Control (IAC) valve
^ Throttle Position Sensor (TPS)

MODES OF OPERATION
The Powertrain Control Module (PCM) uses voltage inputs from several sensors to determine how much fuel to give the engine. The fuel is delivered under several conditions, called "Modes." All modes are controlled by the PCM, and are described below.

Starting Mode
When the ignition is first turned "ON," before engaging starter, the Powertrain Control Module (PCM) energizes the fuel pump relay for two seconds allowing the fuel pump to build up pressure. The PCM then checks the Engine Coolant Temperature sensor (ECT), Manifold Absolute Pressure (MAP), and Throttle Position (TP) 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) operating temperatures. 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 all the way down. The PCM then then completely turns "OFF" the fuel. No fuel is delivered from the injectors as long as the throttle stays wide open, and the engine speed is below 600 rpm. If the throttle position becomes less than 80%, 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 rpm is above 400 rpm, the system goes into Open Loop operation. In Open Loop, the PCM will ignore the signal from the Heated Oxygen Sensor (HO2S), and calculate the air / fuel ratio based on inputs from the ECT, and Intake Air Temperature (IAT) sensors. The system will stay in open Loop until the following conditions are met:

1. Both HO2S have varying voltage output, showing that it is 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.

The specific values for the above conditions vary with different engines, and are stored in the Electrically Erasable Programmable Read Only Memory (EPROM). When the conditions are met, the system goes into Closed Loop operation. In Closed Loop, the PCM will calculate the air / fuel ratio (injector on time) based on the signal from various sensors, but mainly the oxygen sensors. 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 passible 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.

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 6200 rpm with the vehicle in any forward gear, or reverse, and approximately 4000 RPM in park or neutral. To prevent tire damage on vehicles equipped with base tires, the PCM also cuts off fuel from the injectors at vehicle speeds above 161 km/h (100 mph).

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 the PCM shuts "OFF" fuel completely to protect the three-way catalytic converter.

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 enrichens the fuel mixture.

Battery Voltage Correction Mode
When battery voltage is low, the PCM can compensate for the weak spark delivered by the distributor by:
^ Increasing the amount of fuel delivered.
^ Increasing idle rpm.
^ Increasing ignition dwell time.