How Throttle Body Injection Works

Fuel is delivered to the throttle body injection system by an electric fuel pump located in the fuel tank. This pump supplies fuel pressure in sufficient volume to meet the engine's fuel requirement through all load conditions. Injector pressures range from 13 to 16 psi (low pressure system) or 35 to 60 psi (high pressure system), and are kept constant by the fuel pressure regulator. The fuel pressure regulator is a vacuum-operated diaphragm, so that during cold start operation maximum fuel pressure and volume are supplied. Once the engine starts and the manifold vacuum is available, the diaphragm opens a valve and allows unused fuel to return to the tank.

Air is delivered to the engine through the throttle valve. This throttle valve operates like the throttle valve on a carburetor--as the throttle is opened, more air is allowed to enter the engine. Where the throttle body injector differs from the carburetor is in the area of idle control. Instead of calibrated passages, the injector system uses a motorized Idle Air Control (IAC) valve. This valve is in effect a computer-controlled vacuum leak. To increase idle speed, the valve opens to allow more air in. To lower idle speed, the valve closes to allow less air in to the engine.

The engine management system consists of the computer, the injector, the Idle Air Control valve (IAC), the Manifold Absolute Pressure sensor (MAP), the Throttle Position Sensor (TPS), the oxygen sensor(s) and coolant temperature sensor. During cold engine temperatures, the computer operates in open loop mode. In this mode, it monitors engine conditions, but controls fuel and ignition parameters according to preset maps. When the coolant sensor indicates that the engine has reached normal operating temperature and oxygen sensors are warmed up sufficiently, the computer begins closed loop operation. During this time, the computer is controlling fuel and ignition according to sensor inputs from the TPS, MAP and oxygen sensors. As the oxygen sensors readings go lean, the computer adds fuel; as the sensor readings go rich, the computer removes fuel. This is known as the feedback loop. When the throttle is opened, the computer monitor the Throttle Position Sensor to determine the amount of throttle opening. It also monitors manifold vacuum through the Manifold Absolute Pressure Sensor to determine engine load. Based on the indicators, it then advances or retards ignition timing, and increases or decreases injector on time to provide optimum fuel and spark under all driving conditions.