Throttle Body Assembly

The Throttle Body Injection (TBI) system uses the throttle body assembly as a location for mounting the:

* Injectors.

* Fuel metering assembly.

* Pressure regulator.

* IAC motor

* TPS.

Electronic fuel injectors are solenoid-operated valves controlled by the ECM. The injectors are supplied with 12 volts from the ignition relay. Drivers in the ECM supply a ground to the other terminal of the injector to actuate the solenoid.

The ECM energizes the injector solenoid, which opens a ball valve allowing pressurized fuel to flow through a flow director plate that has 6 machined holes creating a conical pattern of finely atomized fuel.

TBI systems fire one injector on one Ref Hi signal and the second injector on the next Ref Hi signal.

The amount of fuel injected is determined by the length of time that the injector is energized, this is known as Pulse Width.

The ECM calibration is based on the fact that an injector discharges a known amount of fuel per millisecond at a specified pressure at a specific voltage. Maintaining proper fuel pressure is ESSENTIAL to providing proper air/fuel ratio.

NOTE:

Do not apply a constant ground directly to the injector’s ground terminal. The internal solenoid will be damaged in a matter of a second or two.

Review the following page for TBI service kits, injector colors and system pressures by model.

Please use this chart whenever servicing GM throttle body units with repair kits. In the 4.3 HUBY and 5.7 MDLK parts boo ks, 3855180 fuel kit is currently incorrect. This kit was originally designed to service the 11 psi throttle body units but now has 29 psi components. 3855180 will be replaced by a variable replacement once the parts department corrects the kits to compensate for both high and low pressure systems.

Kits include:

The regulator is a diaphragm operated relief valve with fuel pressure on one side of the diaphragm and opposing spring pressure on the other.

The regulator’s function is to maintain a constant fuel pressure to the injectors at all times. It is housed inside the throttle body assembly and located in the fuel flow after the injectors.

When fuel pressure exceeds spring pressure a valve will open allowing excess pressure to flow back to the reservoir. The balance between spring pressure,fuel pressure and fuel flow allows the regulator to maintain a constant pressure under all engine operating conditions.