Fuel Delivery and Air Induction: Description and Operation
Fuel Delivery System:
PURPOSE
The purpose of Throttle Body Injection (TBI) is to deliver the correct amount of fuel to the engine under all operating conditions.
CONSTRUCTION
FUEL SYSTEM COMPONENTS:
The fuel supply system is made up of the following components.
1. Fuel pump electrical circuit.
2. Fuel supply components, including:
a. Fuel lines and pipes.
b. Fuel filter
c. Fuel pump.
d. Fuel tank.
3. Throttle Body Injection Assembly, including:
a. Fuel injectors.
b. Fuel pressure regulator.
c. Idle air control valve.
d. Throttle position 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 control module 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 control module, and are described below.
- Starting Mode
When the ignition is first turned "ON," the control module turns "ON" the fuel pump relay for two seconds, and fuel pump builds up pressure. The control module also checks the coolant temperature sensor and the Throttle Position Sensor (TPS) and determines the proper air/fuel ratio for starting. This ranges from approximately 1.5:1 at -36°C (-33°F) to 14.7:1 at 94°C (201°F) engine coolant temperature. The control module controls the amount of fuel delivered by changing how long the injectors are turned "ON" and "OFF." This is done by pulsing the injectors for very short times.
- Clear Flood Mode
If the engine floods, it may be cleared by pushing the accelerator pedal down all the way. The control module then will completely turn "OFF" the fuel. The control module holds this injector rate as long as the throttle stays wide open, and the engine speed is below approximately 637 rpm. If the throttle position becomes less than approximately 75%, the control module 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 637 rpm, the system goes into "OPEN LOOP" operation. In "OPEN LOOP," the control module ignores the signal from the Oxygen sensor, and calculates the air/fuel ratio based on inputs from the coolant temperature and manifold 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 memory calibration module (MEM-CAL). When these conditions are met, the system goes into "CLOSED LOOP" operation. In "CLOSED LOOP," the control module 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 control module responds to rapid changes in throttle position and air flow, and provides extra fuel.
- Deceleration Mode
The control module responds to changes in throttle position and air flow, and reduces the amount of fuel. When deceleration is very fast, the control module can cut off fuel completely for short periods.
- Highway Fuel Mode (Semi-Closed Loop)
This mode comes into operation at highway speeds and it's purpose is to improve fuel economy. For the control module to operate in this mode, it first must sense the correct engine coolant temperature, ignition control, canister purge activity and constant engine speed. During Semi-CLOSED LOOP operation, there will be very little long term fuel trim (formerly known as Block Learn) and short term fuel trim (formerly known as Integrator), and the exhaust sensor values will read below 100 millivolts.
- Decel En-Leanment
On deceleration, the control module senses a high MAP vacuum (low voltage or kPa) and leans the fuel spray for emissions reasons, but it should be noted that the control module can trigger this condition (Decel En-Leanment) while the vehicle is not moving.
- Decel En-Leanment Operation
This mode of operation can be mis-diagnosed as a lean condition. The control module will run the system lean on decel, or if the MAP sensor senses a low voltage (high engine vacuum), with the vehicle standing still, it will lean out the fuel delivery. If it is noted while testing a control module system (With a Bi-directional scanner), and the transmission is in park, that the O2 sensor reading is low (usually below 100 mV) and the long and short term fuel trim are both around the 128 counts, lower the engine speed to 1000 RPM. If the oxygen sensor and long term fuel trim numbers respond normally at this RPM, it's possible that the system was fooled into the decel en-leanment mode of operation. If the oxygen sensor and long term fuel trim numbers do not respond at the lower RPM readings, there are other problems with the vehicle.
- Battery Voltage Correction Mode
When battery voltage is low, the control module can compensate for the weak spark delivered by the Integrated Direct Ignition (IDI) module by:
- Increasing the injector pulse width.
- 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 received from the ignition module, which means the engine is not running. This prevents flooding. Fuel is cut-off at engine speeds above 6900 rpm regardless of throttle position, to prevent engine damage.