Basic operation of a continuous fuel injection is as
Fuel is fed to the system by an electric fuel pump
that delivers fuel to the mixture control unit. A fuel
pressure regulator maintains fuel line pressure and
sends excess fuel back to the tank.
The mixture control unit regulates the amount of
fuel that is sent to the injectors based on the amount of
air flow through the intake and the engine temperature.
The unit is operated by the air flow sensing plate and
The accelerator pedal regulates the rate of air
flow through the intake by opening and closing the
A cold-start injector is installed in the intake to
provide a richer mixture during engine start-up and
warm-up. It is actuated by electric current from the
thermal sensor any time the temperature of the coolant
is below a certain level.
The injector for a continuous fuel injection system
is a simple spring-loaded valve. It injects fuel all the
time the engine is running. A spring holds the valve in a
normally closed position with the engine OFF. This
action keeps fuel from dripping into the engine. When
the engine STARTS, fuel pressure builds and pushes the
injector valve open. A steady stream of gasoline then
sprays toward each intake valve. The fuel is pulled into
the engine when the intake valves open
THROTTLE BODY INJECTION SYSTEM
The throttle body injection (TBI) system (fig. 4-46)
uses one or two injector valves mounted in a throttle
body assembly. The injectors spray fuel into the top of
the throttle body air horn The TBI fuel spray mixes
with the air flowing through the air horn. The mixture is
then pulled into the engine by intake manifold vacuum.
The throttle body injection assembly typically consists
of the following: throttle body housing, fuel injectors,
fuel pressure regulator, throttle positioner, throttle
position sensor, and throttle plates.
The THROTTLE BODY housing, like a
carburetor body, bolts to the pad on the intake manifold.
It houses the metal castings that hold the injectors, the
fuel pressure regulator, and the throttle plates. The
throttle plates are located in the lower section of the
body. A linkage or cable connects the throttle plates
with the accelerator pedal. An inlet fuel line and outlet
return line connects to the fittings on the body.
Figure 4-46.Throttle body injection.