GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide

Description of System


See Figure 1

This is a throttle body fuel injection system with two fuel injectors mounted at the top of the throttle body, which spray fuel down through the throttle valves and into the intake manifold. The throttle body resembles a carburetor in appearance but does away with much of the complexity (choke system, power valves, accelerator pump, fuel circuits, etc.), replacing them with an electronically operated fuel injector.

The injector is actually a solenoid which when activated, lifts a pintle off it's seat, allowing the pressurized fuel behind the valve to spray out. The nozzle of the injector is designed to atomize the fuel for a complete air/fuel mixture.

The activating signal for the injector originates with the Electronic Control Module (ECM), which monitors engine temperature, throttle position, vehicle speed and several other engine-related conditions. The ECM continuously updates the injector opening times in relation to the information given by these sensors.

Fuel pressure for the system is provided by an in-tank fuel pump. The pump is designed to provide fuel at about 18 psi (125 kPa). A fuel pressure regulator in the TBI unit keeps fuel available to the injectors at a constant pressure between 9-13 psi (62-90 kPa). Fuel in excess of injector needs is returned to the fuel tank by a separate line.

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Fig. Fig. 1: TBI operation