The ELECTRIC FUEL PUMP draws fuel out of
the tank and forces it into the pressure regulator.
The FUEL PRESSURE REGULATOR controls
the amount of pressure entering the injector valves.
When sufficient pressure is attained, the regulator
returns excess fuel to the tank. This maintains a preset
amount of fuel pressure for injector valve operation.
The FUEL INJECTOR for an EFI system is a
coil or solenoid-operated fuel valve. When not
energized, spring pressure keeps the injector closed,
keeping fuel from entering the engine. When current
flows through the injector coil or solenoid, the magnetic
field attracts the injector armature. The injector opens,
squirting fuel into the intake manifold under pressure.
The air induction system for the EFI typically
consists of a throttle valve, sensors, an air filter, and
The throttle valve regulates how much air flows
into the engine. In turn, it controls engine power output.
Like the carburetor throttle valve, it is connected to the
gas pedal. When the pedal is depressed, the throttle
valve swings open to allow more air to rush into the
The EFI sensor system monitors engine operating
conditions and reports this information to the computer.
A sensor is an electrical device that changes circuit
resistance or voltage with a change in a condition
(temperature, pressure, position of parts, etc.). For
example, the resistance of a temperature sensor may
decrease as temperature increases. The computer can
use the icreased current flow through the sensor to
calculate any needed change in the injector valve
opening. Typical sensors for an EFI system include the
1. Exhaust gas or oxygen sensor
2. Manifold pressure sensor
3. Throttle position sensor
4. Engine temperature sensor
5. Air flow sensor
6. Inlet air temperature sensor
7. Crankshaft position sensor
Since some of these sensors were discussed in the
section on computerized carburetor systems, we will
only concentrate on the sensors that are particular to the
EFI system. These sensors are as follows:
The THROTTLE POSITION SENSOR is a
variable resistor connected to the throttle plate shaft.
When the throttle swings open for more power or closes
for less power, the sensor changes resistance and signals
the computer. The computer can then enrich or lean the
mixture as needed
The AIR FLOW SENSOR is used in many EFI
systems to measure the amount of outside air entering
the engine. It is usually an air flap or door that operates a
variable resistor. Increased air flow opens the air flap
more to change the position of the resistor. Information
is sent to the computer indicating air inlet volume.
The INLET AIR TEMPERATURE SENSOR
measures the temperature of the air entering the engine.
Cold air is more dense, requiring a little more fuel.
Warm air is NOT as dense as cold, requiring a little less
fuel. The sensor helps the computer compensate for
changes in outside air temperature and maintain an
almost perfect air-fuel mixture ratio.
The CRANKSHAFT POSITION SENSOR is
used to detect engine speed It allows the computer to
change injector openings with changes in engine
The signal from the engine sensors can be either a
digital or an analog type output. Digital signals are on-
off signals. An example is the crankshaft position
sensor that shows engine rpm. Voltage output or
resistance goes from maximum to minimum, like a
switch. An analog signal changes in strength to let the
computer know about a change in condition. Sensor
internal resistance may smoothly increase or decrease
with temperature, pressure, or part position. The sensor
acts as a variable resistor.
Basic operation of an electronic-timed injection
system is as follows:
Fuel is fed by a high-pressure electric fuel pump
to the injectors that are connected in parallel to a
common fuel line.
The fuel pressure regulator is installed in-line
with the injectors to keep fuel pressure constant by
diverting excess fuel back to the tank.
Each injector contains a solenoid valve and is
normally in a closed position. With a pressurized
supply of fuel behind it, each injector will operate
individually whenever electric current is applied to the