Fuel Delivery and Air Induction: Description and Operation

Fuel Metering System:

PURPOSE
The purpose of Port Fuel Injection is to deliver the correct amount of fuel to the engine under all operating conditions.

CONSTRUCTION:
The fuel metering system consists of the following components:

1. Fuel pump electrical circuit.
2. Fuel supply components, including:
a. Fuel lines and pipes.
b. Fuel pump.
c. Fuel tank.
3. Fuel rail assembly, including:
a. Fuel injectors.
b. Fuel pressure regulator.
4. Throttle body assembly, including:
a. Idle air control valve.
b. Throttle position sensor.
c. Mass air flow sensor

OPERATION:
The fuel supply system begins with gasoline in the fuel tank. An electric fuel pump, located in the fuel tank with the fuel level gauge sending unit, pumps fuel to the fuel rail through an in-line filter. The pump is designed to supply fuel at a pressure above the pressure needed by the injectors. A pressure regulator located on the fuel rail keeps the fuel available to the injectors at a constant pressure. The unused fuel is returned to the fuel tank by a separate line.

MODES OF OPERATION:
The ECM 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 ECM, and are described below.

Starting Mode:
When the ignition is first turned "ON," the Electronic Control Module (ECM) turns "ON" the fuel pump relay for two seconds, allowing the fuel pump to build up system pressure. If the engine is immediately started, the fuel pump will continue to operate. If the key is left in the "ON" position, but the engine is not started, the ECM will turn the pump "OFF." The fuel pump will remain "OFF" until the ECM detects the crank signal, or in the case of a faulty fuel pump relay, the oil pressure switch detects approximately 4 psi of oil pressure which will then bypass the fuel pump relay.

Fuel delivery from the port injectors (which deliver fuel in all operating modes) is controlled by changing the amount of time the injectors are turned on or "pulsed" by the ECM. The ECM checks the coolant temperature sensor, throttle position sensor, and the crank signal, and determines 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).

Clear Flood Mode:
If the engine floods, clear it by pushing the accelerator all the way down. The ECM then turns "OFF" the injectors. The ECM holds this injector flow rate as long as the throttle stays wide open and the engine rpm is below 600. If the throttle position becomes less than 65%, the ECM returns to the starting mode.

Run Mode:
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 goes into "OPEN LOOP" operation. In "OPEN LOOP," the ECM ignores the signal from the Oxygen sensor, and calculates the air/fuel ratio based on inputs from the coolant temperature, throttle position, and the mass air flow (MAF) sensors.

The system will stay in "OPEN LOOP" until the following conditions are met:
1. The Oxygen sensor has a varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature.)
2. The Coolant temperature is above a specified temperature.
3. A specified amount of time has elapsed after starting the engine.

CLOSED LOOP: The specified operating conditions vary with different engines, and are stored in the memory calibration module (Mem-Cal). When these conditions are met, the system goes into "CLOSED LOOP" operation. In "CLOSED LOOP," the ECM calculates the air/fuel ratio (injector on-time) based on the signal from the Oxygen sensor. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration Mode:
The ECM responds to rapid changes in throttle position and air flow, and provides extra fuel.

Deceleration Mode:
The ECM responds to changes in throttle position and air flow, and reduces the amount of fuel. When deceleration is very fast, the ECM can cut off fuel completely for short periods.

Battery Correction Mode:
When battery voltage is low, the ECM can compensate for the weak spark by:
1. Increasing the amount of fuel delivered.
2. Increasing the idle speed.
3. Increasing the ignition dwell time.