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 (TP) Sensor
- Mass Air Flow (MAF) Sensor

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), Throttle Position (TP) sensor and crankshaft position signal to determine the proper air / fuel ratio for starting.
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 TP, ECT, and MAF sensors. The system will stay in open Loop until the following conditions are met:

1. The HO2S has 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.
4. Engine speed have been greater than 800 RPM since start up.

The specific values for the above conditions vary with different engines, and are stored in the Programmable Read Only Memory (PROM). 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.

Deceleration Mode
The PCM reduces the amount of fuel injected when it detects a decrease in throttle position and air flow. When deceleration is very fast, the PCM may cut off fuel completely for short periods.

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.

Fuel Cutoff Mode
No fuel is delivered by the injectors when the ignition is "OFF." This prevents dieseling. In addition, the PCM suspends fuel delivery if no reference pulses are detected (engine not running) to prevent engine flooding.


Engine Speed / Vehicle Speed Fuel Disable Mode
The PCM monitors engine speed and turns "OFF" the fuel injectors when engine speed increases above 5600 RPM. Fuel is turned back "ON" when engine speed decreases below 5100 RPM.
The PCM also monitors vehicle speed and turns "OFF" the fuel injectors at a vehicle speed which varies according to axle ratio, tire size and rating.