A Throttle Body Assembly is an integral part of a fuel-injected engine’s air intake system. Its function is to regulate the amount of air that flows into the engine – accounting for factors such as throttle (gas pedal) position, idle speed, cold start warmup, and more. To perform these critical tasks, the throttle body is bolted in place between your engine’s intake manifold and air filter housing.
A hinged metal plate inside the throttle body (known as a butterfly valve) opens to let more air in as the gas pedal is depressed. Because fuel injection systems are inherently computer controlled, information from various sensors mounted on the throttle body allows the vehicle’s engine management system to determine the optimum fuel flow rate based on multiple conditions.
Throttle body assemblies are known as either "mechanical" or "electronically controlled". On a mechanical throttle body (above left) the butterfly valve is operated by a spring-loaded arm piece that's directly linked to a vehicle's accelerator pedal by way of a manual cable. Newer vehicles that do not have cable linkages to the accelerator pedal will use throttle body assemblies with computer-controlled servo motors to move the butterfly valve as needed (above right). These servo motor assemblies are described as electronic throttle controls.
On either setup, a knob on the outside of the housing rotates as the butterfly valve moves in order to give feedback to an attached throttle position sensor. This sensor then sends an electronic signal to the ECU (Engine Control Unit) so that fuel flow is adjusted to match the amount of air that's now entering the engine.
On the outside of the throttle body assembly will be a hole for mounting a Manifold Absolute Pressure (MAP)sensor. This port connects to a small air duct that runs into the pressure chamber behind the throttle plate. As more air flows in, the MAP sensor will read a higher pressure, which the ECU uses to verify calculations for fuel flow rate.