to the mixture control unit by the fuel pump. The fuel
speed up the engine idle during warm-up.
It also is
pressure regulator maintains fuel line pressure by
actuated by the thermal sensor.
sending excess fuel back to the gas tank. The mixture
control unit regulates the amount of fuel that is sent to
4-28. Throttle Body Injection (Fig. 4-66). Throttle
the injectors based on the amount of airflow through the
body injection is a form of continuous injection that uses
intake and the engine temperature. The
one or two injectors delivering gasoline to the engine
control unit on mechanical systems will be operated by
from one central point in the intake manifold. Though
the airflow sensing plate and the warm-up regulator.
throttle body injection does not provide the precise fuel
This information on electronic systems will be fed into a
distribution of the direct port injection described in the
computer that will, in turn, regulate the fuel injection rate.
previous paragraph, it is much cheaper to produce, yet
The accelerator pedal will regulate the rate of airflow
provides a much higher degree of precision fuel metering
through the intake by opening and closing the throttle
than a carburetor. The throttle body injection unit is
valve. A cold-start injector is installed in the intake to
usually an integral one, containing all of the major
provide a richer mixture during engine startup and warm-
system components, in most cases. The unit mounts on
up. It is actuated by electric current from the thermal
the intake manifold in the same manner as a carburetor.
sensor whenever the temperature of the coolant is below
Airflow sensors and electronic computers usually are
a certain level.
The cold-start injector works in
mounted in the air cleaner body.
conjunction with the auxiliary air valve. Its function is to
Figure 4-66. Throttle Body Injection.