What Is a Throttle Body?

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Internal combustion gasoline engines require three things to operate: oxygen, fuel and a spark. On modern engines, fuel delivery and spark timing are handled by a computer controlled system of pumps and actuators. Air delivery is a bit more analog, which is where the throttle body comes in.

Fuel delivery is computer-controlled on modern car engines
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Throttle bodies are essentially air valves, and can take a number of forms. Most are butterfly valves, but barrel-type and pinch valves have also been used.

Man working on engine
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Prior to fuel injection, air delivery was modulated by a set of butterfly valves set into the base of an engine's carburetor.

Open hotrod engine
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Most engines use Multi-Point Fuel Injection (MFI.) In these systems, the throttle body is responsible only for modulating air delivery.

Car engine
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Many older engines use Throttle Body Injection (TBI.) These throttle bodies resemble carburetors, and utilize one or a pair of fuel injectors mounted atop the assembly, which shoot fuel through the butterfly valves and into the engine.

Mechanic working on engine
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On most engines, the throttle body also contains a secondary circuit to control airflow at idle. This channel is called the IAC (Idle Air Control), and utilizes a computer-controlled valve to vary airflow.

Mechanic working on engine of car
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