Computers and Control Systems: Description and Operation

Fig. 1 TBI Unit:

IAC Valve Operation And Fuel Pressure Regulation:

TBI Unit:

The Throttle Body Fuel Injection System provides 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.

The ECM is capable of diagnosing faults with the various inputs and systems it controls. When the ECM detects a malfunction, a "Check Engine" or "Service Engine Soon" lamp in the instrument panel is illuminated, warning the driver to have the system checked.

In the fuel injection system used with 4 cylinder engines, Fig. 1, 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, Fig. 2. 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, Fig. 2.

On fuel injected V6 and V8 applications, Fig. 3, the throttle body injector (TBI) is also located on the intake manifold where fuel and air are distributed through a double bore throttle body, Fig. 2. Combustion air is controlled by dual throttle valves while fuel is supplied by dual TBI unit mounted injectors. Both injectors are controlled by one ECM.

When the ignition key is turned on, the ECM will energize the fuel pump relay. The fuel tank mounted 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.

If a fuel pump relay failure should occur, the oil pressure switch can also operate the fuel pump after the engine has built up approximately 4 psi of oil pressure.