IN MANY FUEL INJECTED ENGINES, THE THROTTLE BODY IS THE PORTION OF THE AIR INTAKE SYSTEM THAT REGULATES THE AMOUNT OF AIR FLOWING INTO THE ENGINE, IN REACTION TO THE DRIVER ACCELERATOR PEDAL RESPONSE IN THE MAIN. THE THROTTLE BODY IS GENERALLY LOCATED BETWEEN THE AIR FILTER BOX AND THE INTAKE MANIFOLD, AND IT IS COMMONLY ATTACHED TO, OR NEAR, THE MASS AIRFLOW SENSOR. THE BIGGEST PIECE INSIDE THE THROTTLE BODY IS THE THROTTLE PLATE, WHICH IS A BUTTERFLY VALVE THAT CONTROLS THE AIRFLOW.
The accelerator pedal motion is converted via the throttle cable, to activate the throttle linkages, which move the throttle plate. In cars with electronic throttle control, an electric motor regulates the throttle linkages and the accelerator pedal connects not to the throttle body, but to a sensor, which directs the pedal position to the Engine Control Unit (ECU). The ECU determines the throttle opening based on accelerator pedal position and records from other engine sensors. Throttle bodies may also contain valves and adjustments to direct the minimum airflow during idle. Even in those units that are not "drive-by-wire", there will often be a tiny electric motor driven valve, the Idle Air Control Valve (IACV), which the ECU uses to control the amount of air that can bypass the main throttle opening.
Carburetors combine the functionality of the throttle body and fuel injectors into one in order to moderate the amount of airflow and to combine air and fuel together. Cars with throttle body injection (called TBI by General Motors and CFI by Ford) locate the fuel injectors in the throttle body, thereby allowing an older engine to be changed from carburetor to fuel injection without significantly altering the engine design.