BASIC OPERATION
The fuel metering system consists of the following components:
Fuel pump electrical circuit.
Fuel supply components, including:
Fuel lines and pipes.
Fuel pump.
Fuel tank.
Throttle Body Injection Assembly, including:
Fuel injectors.
Fuel pressure regulator.
Idle air control valve.
Throttle position sensor.
BASIC SYSTEM 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 throttle body assembly 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 in the throttle body assembly 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. On Heavy Duty models, a fuel module will override the ECM two second timer and the fuel pump will run for twenty seconds and then shut OFF if the vehicle is not started. This circuit corrects a hot restart (vapor lock) during high ambient temperature conditions. 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 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 (CTS), Throttle Position Sensor (TPS), 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, it may be cleared by pushing the accelerator pedal down all the way. The ECM then pulses the injector at a 20:1 air fuel ratio. The ECM holds this injector rate as long as the throttle stays wide open, and the engine speed is below approximately 600 rpm. If the throttle position becomes less than approximately 80%, 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 and Manifold Absolute Pressure (MAP) sensors.
The system will stay in OPEN LOOP until the following conditions are met:
The oxygen sensor has a varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature)
The coolant temperature is above a specified temperature.
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 programmable read only memory (PROM). 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 manifold pressure or air flow, and provides extra fuel.
Deceleration Mode:
The ECM responds to changes in throttle position and manifold pressure 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:
Increasing the amount of fuel delivered.
Increasing the idle speed.
Increasing the ignition dwell time.
Fuel Cut-off Mode:
No fuel is delivered by the injector when the ignition is OFF. This prevents dieseling. Also, fuel is not delivered if no reference pulses are seen from the distributor, which means the engine is not running. This prevents flooding. Fuel cut-off also occurs at high engine rpm, to protect internal engine components from damage
Monday, August 7th, 2006 AT 2:29 PM
