GM Lumina/Grand Prix/Cutlass Supreme/Regal 1988-1996

System Description


See Figures 1 and 2

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Fig. Fig. 1: Exploded view of the TBI model 700 assembly which is used on 2.5L engines

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Fig. Fig. 2: Schematic of the TBI 700 fuel flow

The electronic Throttle Body Injection (TBI) system, used on 2.5L engines, is a fuel metering system with the amount of fuel delivered by the throttle body injector determined by an electronic signal supplied by the Electronic Control Module (ECM). The ECM monitors various engine and vehicle conditions to calculate the fuel delivery time (pulse width) of the injector. The fuel pulse may be modified by the ECM to account for special operating conditions, such as cranking, cold starting, altitude, acceleration, and deceleration.

The TBI system provides a means of fuel distribution for controlling exhaust emissions within legislated limits. The TBI system, by precisely controlling the air/fuel mixture under all operating conditions, provides as near as possible complete combustion.

This is accomplished by using an Electronic Control Module (ECM) (a small on-board microcomputer) that receives electrical inputs from various sensors about engine operating conditions. An oxygen sensor in the main exhaust stream functions to provide feedback information to the ECM as to the oxygen content, lean or rich, in the exhaust. The ECM uses this information from the oxygen sensor, and other sensors, to modify fuel delivery to achieve, as near as possible, an ideal air/fuel ratio of 14.7:1. This air/fuel ratio allows the 3-way catalytic converter to be more efficient in the conversion process of reducing exhaust emissions while at the same time providing acceptable levels of driveability and fuel economy.

The basic TBI model 700 is made up of 2 major casting assemblies: (1) a throttle body with a valve to control airflow and (2) a fuel body assembly with an integral pressure regulator and fuel injector to supply the required fuel. A device to control idle speed (Idle Air Control Valve) and a device to provide information about throttle valve position (Throttle Position Sensor) are included as part of the TBI unit.