A throttle body is a device that is used to monitor the air flow in a fuel-injected engine. Unlike a carburetor, it does not provide fuel in the air mixture. The throttle-body-equipped engine uses fuel injectors to provide fuel to the engine. Typically equipped with carburetor-like throttle butterflies, the throttle body meters air into the engine according to how far the operator is pushing the throttle pedal down. As the pedal is depressed, the butterflies open further, allowing more air into the engine while the vehicle's computer signals the fuel injectors to spray more fuel into the engine.
On some throttle body assemblies, throttle position sensors are used to meter the fuel passing through the fuel injectors. Fuel injector units do not have a choke mechanism to aid in cold-weather starts, so the fuel injection units use a high-speed sensor to spray extra fuel while holding the throttle body closed. This functions much like a choke on a carburetor and allows the engine to run at a richer fuel setting until it is warm. The computer on the vehicle will control this entire process. Unlike a carburetor that requires a couple of stiff pumps to engage the choke, the vehicle operator need not do anything other than turn the key for a cold-weather start-up.
The throttle body does not require an intense maintenance regimen. Usually, keeping a clean air filter in place along with occasionally spraying the assembly with a quality cleaning solvent at oil change intervals will keep the unit in tip-top operating condition. Occasionally the fuel injectors will require cleaning as well, and using a quality injector cleaner will return them to peak operational condition. For the best treatment of the engine and the fuel injection system, the engine should never be operated without an air filter in place.
For vehicles originally equipped with a carburetor, several companies manufacture and sell fuel-injection kits complete with a throttle body. These kits allow the older engine to run more efficiently and, in many cases, provide the operator with better performance and fuel mileage. Most of the aftermarket conversion kits come complete with computer, fuel injectors and throttle body assembly as well all necessary mounts, bolts and wiring harnesses. In most applications, the installation is very simple, with removal and swap of the vehicle's carburetor making up the bulk of the installation.