In the continuous system, fuel is delivered to the
mixture control unit by the fuel pump. The fuel pressure
regulator maintains fuel line pressure by sending excess
fuel back to the gas tank.
The mixture control unit regulates the amount of
fuel that is sent to the injectors, based on the amount of
airflow through the intake and the engine temperature.
The mixture control unit on mechanical systems is
operated by the airflow sensing plate and the warm-up
regulator. This information on an electronic system is
fed into a computer that regulates the fuel injection rate.
The accelerator pedal regulates the rate of airflow
through the intake by opening and closing the throttle valve.
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
whenever the temperature of the coolant is below a certain
level. The cold-start injector works in conjunction with the
auxiliary air valve. Its function is to speed up the engine idle
during warm-up. It is also actuated by the thermal sensor.
THROTTLE BODY INJECTION SYSTEMS
Throttle body injection (fig. 5-4) is a form of continuous
injection-one or two injectors delivering gasoline to the
engine from one central point in the intake manifold. Though
throttle body injection does not provide the precise fuel
distribution of the direct port injection, it is cheaper to
produce and to provide a degree of precision fuel metering.
The throttle body injection unit is usually an integral one and
contains all of the major system components. The unit
mounts on the intake manifold in the same manner as a
carburetor. Airflow sensors and electronic
usually are mounted in the air cleaner body.
SERVICING AND PRECAUTIONS
When a vehicle equipped with a gasoline type of
fuel injection system has a problem, check all other
systems first, such as ignition, air intake, charging,
exhaust systems, and so forth-before you work on the
fuel injection system. The fuel injection system is
usually the last (least problematic) system to cause
trouble. There are servicing precautions you should
observe before you work on gasoline fuel injection
1. Do not jump the battery to start the vehicle.
2. Do not disconnect the battery cables from the
battery with the engine running.
3. When charging a battery in the vehicle,
disconnect the negative (grounded) terminal.
4. Do not remove or attach the wiring harness plug
to the electronic control unit (computer) with the
5. Before performing a compression test, check the
manufacturers repair manual for special instructions.
6. Always make sure all other systems are in good
working order before you adjust or troubleshoot the
gasoline fuel injection system.
These precautions are general and apply to most
systems. Nevertheless, use good judgment, and always
Figure 5-4.-Throttle body injection.