The 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.

When 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 commonly used.

The 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.

When 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.

The 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.

Fuel 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.

There 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.

Some 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 temperature.

When 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 mixture.

A 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.

One 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).

The 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.