System Operation

Fig. 1 Throttle Body Injector Unit:

THROTTLE BODY INJECTOR UNIT VIEW

Fig. 2 Fuel Metering Schematic:

FUEL SYSTEM SCHEMATIC



The Throttle Body Fuel Injection Systems provide a means of fuel distribution for controlling exhaust emissions within the required limits by precisely controlling the air/fuel ratio under all operating conditions. This is accomplished by means of an Electronic Control Module (ECM) which receives electrical signals from various sensors indicating engine operating conditions, and varies the fuel delivery time (pulse width) of the injector(s) accordingly. The ECM may modify the fuel pulse to compensate for special operating conditions such as cranking, cold starting, altitude, acceleration and deceleration. By increasing the injector pulse, more fuel is delivered and the air/fuel mixture is enriched. When the injector pulse is decreased, the air/fuel ratio is decreased.
In the fuel injection system used with the 4 cylinder engines, the throttle body injector (TBI) is located on the intake manifold where fuel and air are distributed through a single bore in the throttle body. Air for combustion is controlled by a single throttle valve which is connected to the accelerator pedal linkage by a throttle shaft and lever assembly. Fuel for combustion is supplied by a single fuel injector mounted on the TBI assembly.
The fuel injection system used on 6 cylinder engines, is similar to the system used on 4 cylinder engines except that the throttle body injection unit on 6 cylinder engines has two fuel injectors.
On fuel injected V8 applications, the ``2 1'' or ``Crossfire'' fuel injection system is used. This system incorporates a front and rear TBI unit, each controlled by an ECM. Each TBI unit supplies the correct air/fuel mixture through long runners in the intake manifold to the bank of cylinders located on the opposite side of the engine. A fuel pressure compensator is located on the fuel meter cover of the front unit. The compensator will equalize fuel pressure between the two TBI units during the momentary pressure drop which may occur at injector ``on'' time. The TBI units in both systems utilize a special ``swirl'' plate located directly beneath the throttle valve to aid in mixture distribution.
When the ignition key is turned on, the ECM will energize the fuel pump relay. The fuel pump pressurizes the system to approximately 10 psi. If a distributor reference pulse is not received by the ECM within two seconds, the fuel pump relay will be de-energized, turning off the fuel pump. If a distributor reference pulse is later received by the ECM, the fuel pump relay will be re-energized and the pump will resume operation.
The Throttle Body Fuel Injection systems are comprised of the following major sub-systems: Fuel Supply, Throttle Body Injector Assembly (TBI), Idle Air Control (IAC), Data Sensors, Electronic Control Module (ECM), Electronic Spark Timing (EST), and, on 6 and 8 cylinder engines, Electronic Spark Control (ESC).