GM Metro/Sprint 1985-1993 Repair Guide



See Figures 1 through 6

Electronic throttle body fuel injection (TBI) is a fuel metering system which provides a means of fuel distribution for controlling exhaust emissions. By precisely controlling the air/fuel mixture under all operating conditions, the system provides as near as possible complete combustion.

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Fig. Fig. 1: Throttle body cross-section

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Fig. Fig. 2: Fuel pump cross-section

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Fig. Fig. 3: Fuel injector cross-section

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Fig. Fig. 4: Pressure regulator cross-section

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Fig. Fig. 5: Air valve cross-section

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Fig. Fig. 6: Disconnecting the fuel pump relay

The amount of fuel delivered by the throttle body injector is determined by a 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(s). The fuel pulse may be modified by the ECM to account for special operating conditions, such as cranking, cold starting, altitude, acceleration, and deceleration.

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.

A single fuel injector is mounted on top of a throttle body, which replaces the carburetor on the intake manifold. The injector is a solenoid-operated device controlled by the ECM. The incoming fuel is directed to the lower end of the injector assembly which has a fine screen filter surrounding the injector inlet. The ECM turns on the solenoid, which lifts a normally closed ball valve off a seat. The fuel, under pressure, is injected in a conical spray pattern at the walls of the throttle body bore above the throttle valve. The excess fuel passes through a pressure regulator before being returned to the vehicle fuel tank.

The pressure regulator is a diaphragm-operated relief valve with the injector pressure on one side, and the air cleaner pressure on the other. The function of the regulator is to maintain constant pressure (approximately 11 psi) to the injector throughout the operating loads and speed ranges of the engine. If the regulator pressure is too low, below 9 psi, it can cause poor performance. Too high a pressure could cause detonation and a strong fuel odor.

The fuel pump is mounted in the fuel tank. This electrically operated pump supplies sufficient pressure (25-33 psi) to provide proper fuel atomization at the injector. The fuel pump also uses a check valve to hold fuel pressure within the fuel feed line when the vehicle is not running.

A relay is used to control voltage to the fuel pump. When the ignition key is turned ON , the ECM will initialize (start program running) and energize the fuel pump relay. The fuel pump pressurizes the system to approximately 10 psi. If the ECM does not receive a distributor reference pulse (telling the ECM the engine is turning) within 2 seconds, the ECM will then de-energize the fuel pump relay, turning off the fuel pump. If a distributor reference pulse is later received, the ECM will turn the fuel pump back on.

An air valve is used to control intake air during cold start conditions. The air valve consist of thermo-wax, springs and a valve. When the engine is cold, the thermo-wax contracts. In this state, the valve opens by the spring force, allowing the air to be drawn into the intake manifold. Thus the amount of intake air increases even when throttle valve is at idle position and engine speed rises to the fast idle state.