Throttle Body: Description and Operation

Fig. 1 Front view of fuel charging assembly:

Fig. 2 Rear view of fuel charging assembly:

Fig. 5 High pressure fuel pump wiring circuit (Typical):

The throttle body consists of six separate components which perform the fuel and air metering to the engine. The throttle body is mounted on the intake manifold and provides a housing for the 1) Air Control Butterfly Valves, 2) Injector Nozzles, 3) Fuel Pressure Regulator, 4) Cold Engine Speed Control), 5) Throttle Position Sensor and 6) Fuel Pressure Diagnostic Valve.
The air flow is controlled by two butterfly valves which are mounted on the throttle body and are similar in design and operation to those of a carburetor.
The fuel injector nozzles are mounted vertically above the throttle plates. They are electro-mechanical units that meter and atomize the fuel delivered to the engine, Figs. 1, 2 and 5. The injector consists of a solenoid actuated pintle and needle valve assembly. An electrical control signal from the EEC electronic processor actuates the solenoid causing the pintle to move off its seat allowing fuel flow. The injector flow orifice is fixed and the fuel supply pressure is constant, therefore fuel flow to the engine is determined by the amount of time the solenoid is energized.
The pressure regulator, Figs. 1 and 2, is mounted on the fuel charging main body near the rear of the air horn. The regulator is located so as to offset the affects of pressure fluctuations in the fuel system. It is designed so that it is not affected by back pressure in the fuel return line.
The pressure regulator maintains a working pressure of 14.5 psi and also maintains a fuel supply pressure after engine and fuel pump turn off, by acting as a check valve between itself and the fuel pump. This maintenance of pressure helps to prevent fuel line vapor lock and permits rapid restarts and stable engine idling immediately thereafter.
The diagnostic pressure valve located at the top of the fuel metering body provides a convenient point to monitor fuel pump pressure and allows for bleeding down of the system pressure and bleeding air during service. Do not apply compressed air to the fuel system through the diagnostic valve.

Fig. 12 Cold engine speed control:

Cold engine speed control is accomplished by a throttle stop cam positioner similar to that used on carburetors, Fig. 12. The cam is positioned by a bi-metal spring and an electric positive temperature heating element. The electrical source for the heating element is 7.3 volts from the alternator which provides voltage only when the engine is running. The heating element provides the necessary warm up according to starting temperature (cold engine) and the length of time after starting. Multiple positions on the cam allow for a decreasing cold engine speed to curb idle speed during warm up. The cold engine speed control also provides an automatic kick down from high cam (fast idle) engine speed to some intermediate speed. This is accomplished through a vacuum motor and bleed which physically moves the cam a short time after starting the engine.