fuel injector is an electromechanical device that meters and atomizes fuel so it
can be sprayed into the intake manifold. Fuel injectors resemble spark plugs in
size and shape. As mentioned earlier, they mount into the intake manifold
runners (port systems) or throttle body housing (TBI systems). O-rings are used
to seal the injector at the intake manifold, throttle body, and fuel rail
mounting positions. These O-rings provide thermal insulation to prevent the
formation of vapor bubbles and promote good hot start characteristics. They also
dampen potentially damaging vibration.
the injector is electrically energized, a solenoid-operated valve opens, and a
fine mist of fuel sprays from the injector tip. Two different valve designs are
first consists of a valve body and a nozzle or needle valve that has a special
ground pintle. A movable armature is attached to the nozzle valve, which is
pressed against the nozzle body sealing seat by a helical spring. The solenoid
winding is located at the back of the valve body.
the solenoid winding is energized, it creates a magnetic field that draws the
armature back and pulls the nozzle valve from its seat. When the solenoid is
de-energized, the magnetic field collapses and the helical spring forces the
nozzle valve back on its seat.
second popular valve design uses a ball valve and valve seat. In this case, the
magnetic field created by the solenoid coil pulls a plunger upward lifting the
ball valve from its seat. Once again, a spring is used to return the valve to
its seated or closed position.
injectors can be either top fuel feeding or bottom fuel feeding. Top feed
injectors are primarily used in port injection systems that operate using high
fuel system pressures. Bottom feed injectors are used in throttle body systems.
Bottom feed injectors are able to use fuel pressures as low as 10 psi.
have been some problems with deposits on injector tips. Since small quantities
of gum are present in gasoline, injector deposits usually occur when this gum
bakes onto the injector tips after hot engine is shut off. Most oil companies
have added a detergent to their gasoline to help prevent injector tip deposits.
Car manufacturers and auto parts stores sell detergents to place in the fuel
tank to clean injector tips.
manufacturers and auto parts suppliers have designed deposit-resistant
injectors. These injectors have several different pintle tip and orifice designs
to help prevent deposits. On one type of deposit-resistant injector, the pintle
seat opens outward away from the injector body and more clearance is provided
between the pintle and the body. Another type of deposit-resistant injector has
four orifices in a metering plate rather than a single orifice.
COLD START INJECTOR A good number of engines equipped with port injection
have an additional injector, the cold start injector. Unlike the individual
injectors at the intake ports, the cold start injector is not operated directly
by the ECU. Rather, it is opened by a thermo-time switch, which senses coolant
the engine is cranked, voltage is supplied from the starter solenoid to one
terminal on the cold start injector. If the coolant temperature is below a
certain amount, the thermo-time switch completes the ground for the cold start
injector. Doing this energizes the injector while the engine is cranking. The
fuel from the cold start injector sprays into the intake manifold and is
delivered to the cylinders. Since the injectors at each intake port will also be
providing fuel, the fuel from the cold start injector allows for a richer
bimetal switch in the thermo-time switch is heated as current flows through the
injector coil. The bimetal switch action opens the circuit through the
thermo-time switch after the engine has reached a certain temperature.
ELECTRICAL CONNECTOR Each fuel injector is equipped with a two-wire
connector. The connector is often equipped with a spring clip that must be
unlocked before the connector can be removed from the injector.
wire of the connector supplies voltage to the injector. This voltage supply wire
may connect directly to the fuse panel. It may connect to the ECU, which, in
turn, connects to the fuse panel. In some systems, a resistor at the fuse panel
or ECU is used to reduce the 12-volt battery supply voltage to 3 volts or less.
Most other injectors are fed battery voltage (12-volts).
second wire of the connector is a ground wire. This ground wire is routed to the
ECU. The ECU energizes the injector by grounding its electrical circuit. The
pulse width of the injector equals the length of time the injector circuit is
grounded. Typical pulse widths range from 1 millisecond to 10 milliseconds at
full load. Port fuel injection systems having four, six, or eight injectors use
a special wiring harness to simplify and organize injector wiring.