Multipoint - EFI System (Part7. THROTTLE BODY ASSEMBLY)

The throttle body assembly for most modern fuel in­jection systems is used only to control air flow into the engine. A throttle valve, like the butterfly valve in a carburetor, is attached to the throttle cable and gas pedal. When the driver presses the gas pedal, the shaft rotates. This swings the throttle valve open to admit more air. This increases engine power for acceleration or pulling a load. The throttle body, itself, is usually made of cast aluminum. It mounts in the induction system just ahead of the intake manifold. As was shown in Fig below, it sometimes bolts to the inlet of the intake manifold.

Throttle body assembly only controls air flow to engine with most modern fuel injection systems. (Ford)

There are many throttle body designs for fuel in­jected systems. Two variations are shown here. (Ford and Honda)

Air Bypass Valve (idle speed control valve)

An air bypass valve, also known as an idle speed con­trol valve, is frequently used to regulate engine idle speed. The air bypass valve normally mounts on the throttle body assembly. It can be controlled by either a temperature-sensitive device or by the computer. In the figure shows the action of the temperature (bimetal strip) operated idle air control valve. When the control valve and engine are cold, the bimetal strip holds. the air bypass open. This increases engine speed. When the engine warms, the bimetal strip bends and moves to close the idle air valve. This drops engine idle speed to normal.
The picture illustrates the basic action of a computer controlled idle air valve. When the engine is cold, the engine temperature sensor signals the computer. The computer knows that engine rpm should be increased to prevent engine stalling and stumbling. It then sends current to the idle air control valve. This opens the valve and allows air to bypass the throttle valve. The rest of the injection systems react to this increased air flow and engine idle speed increases, just as a carburetor fast idle cam increases cold engine idle rpm.
As the engine warms, the computer operates the idle air control valve to close off the bypass. Then, no extra air flows around the throttle plate and idle speed returns to normal again.

Air bypass valve acts like fast idle mechanism on carbureted fuel system. It speeds up engine idle rpm when it is cold. (Volkswagen)

Expansion and contraction of metals in bimetallic strip open and close air door in this air bypass valve. When engine is cold, metal contracts and opens door admitting more air. As engine warms, warm air, assisted by electric heat element, closes blocking plate to lower idle speed. (Renault and AMC)

shows a cutaway view of one computer controlled type of idle speed valve. When the engine is cold, the computer sends current to rotate the small dc motor in one direction. This turns the rotor and screw to pull the air bypass valve open. When the engine warms, the computer reverses the polarity to the motor so it turns the screw in the opposite direction, closing the valve.

Cutaway of idle air control valve operated by small dc motor and computer. Current from computer energizes motor which turns threaded shaft to open and close air valve. (Toyota)