GMC T Series 1994 Repair Manual page 568

The fuel control system has an electric fuel pump,
located in the fuel tank with the gage sending unit, that
pumps fuel to the TBI unit through the fuel supply line,
then through an in-line fuel filter. On vehicles with two
fuel tanks, there is an electric fuel pump and gage
sending unit in each fuel tank.
A pressure regulator in the TBI unit keeps fuel avail­
able to the injectors at a constant pressure between 62
and 90 kPa (9 and 13 psi). If the vehicle is equipped
with a 7.4L engine, pressure is kept between 179 and
220 kPa (26 and 32 psi). Fuel in excess of injector
needs is returned to the fuel tank by a separate line.
The ECM/PCM monitors information from several
sensors to determ ine fuel requirem ents based on
engine operating conditions. The fuel is delivered under
one of several conditions called " modes." All the modes
are controlled by the ECM/PCM.
Model 220 TBI Unit
Model 220, used on V6 and V8 engines, consists of
three major casting assemblies:
• Fuel meter cover with pressure regulator.
• Fuel meter body with fuel injectors.
• Throttle body with:
— Idle air control (IAC) valve.
— Throttle position sensor (TP sensor).
— Idle speed control (ISC) actuator (7.4L only).
GENERAL DESCRIPTION
Identification
The throttle body identification number is stamped
vertically on the throttle body next to the vacuum ports
(figure 1). Refer to this number when servicing the
throttle body. When replacing the throttle body injection
unit, transfer the identification number to the new throt­
tle body injection unit.
For a disassembled component view of the throttle
body injection unit, refer to figure 2.