See Figure 1
The Throttle Body Injection (TBI) system is a fuel metering system in which the amount of fuel delivered by the throttle body injectors is determined by an electronic signal supplied by the Electronic Control Module (ECM). The ECM monitors various engine and vehicle conditions to calculate the fuel delivery time (pulse width) of the injectors. The fuel pulse may be modified by the ECM to account for special operating conditions, such as cranking, cold starting, altitude, acceleration, and deceleration.
The ECM controls the exhaust emissions by modifying fuel delivery to achieve, as near as possible, an air/fuel ratio of 14.7:1. The injector on-time is determined by various inputs to the ECM. By increasing the injector pulse, more fuel is delivered, enriching the air/fuel ratio. Decreasing the injector pulse leans the air/fuel ratio.
Pulses are sent to the injectors in 2 different modes: synchronized and non-synchronized. In synchronized mode operation, the injectors are pulsed alternately. In non-synchronized mode operation, the injectors are pulsed once every 12.5 milliseconds or 6.25 milliseconds depending, on calibration. This pulse time is totally independent of distributor reference pulses. Non-synchronized mode results only under the following conditions:
The basic TBI unit is made up of 2 major casting assemblies: (1) a throttle body with a valve to control airflow and (2) a fuel body assembly with an integral pressure regulator and fuel injectors to supply the required fuel. An electronically operated device to control the idle speed, and a device to provide information regarding throttle valve position, are included as part of the TBI unit.
Each fuel injector is a solenoid-operated device controlled by the ECM. The incoming fuel is directed to the lower end of the injector assembly, which has a fine screen filter surrounding the injector inlet. The ECM actuates the solenoid, which lifts a normally closed ball valve off a seat. The fuel under pressure is injected in a conical spray pattern at the walls of the throttle body bore, above the throttle valve. The excess fuel passes through a pressure regulator before being returned to the vehicle's fuel tank.
The pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and air cleaner pressure on the other. The function of the regulator is to maintain a constant pressure drop across the injectors throughout the operating load and speed range of the engine.
The throttle body portion of the TBI unit may contain ports located at, above or below the throttle valve. These ports generate the vacuum signals for the EGR valve, MAP sensor, and the canister purge system.
The Throttle Position Sensor (TPS) is a variable resistor used to convert the degree of throttle plate opening into an electrical signal for the ECM. The ECM uses this signal as a reference point of throttle valve position. In addition, an Idle Air Control (IAC) assembly, mounted in the throttle body, is used to control idle speeds. A cone-shaped valve in the IAC assembly is located in an air passage in the throttle body that leads from the point beneath the air cleaner to below the throttle valve. The ECM monitors idle speeds and, depending on engine load, moves the IAC cone in the air passage to increase, or decrease air bypassing the throttle valve to the intake manifold for control of idle speeds.