GM Cavalier/Sunbird/Skyhawk/Firenza 1982-1994

System Description


The single bore, Model 300 throttle body unit used on the 1983-86 1.8L OHC engine and the Model 500 throttle body unit used on the 1982-86 2.0L OHV engine are similar systems.

In these throttle body systems, a single fuel injector mounted at the top of the throttle body sprays fuel down through the throttle valve and into the intake manifold. The throttle body resembles a carburetor in appearance but does away with much of the carburetor's complexity (choke system and linkage, power valves, accelerator pump, jets, fuel circuits, etc.), replacing these with the electrically operated fuel injector.

The injector is actually a solenoid which when activated lifts a pintle valve off its seat, allowing the pressurized (10 psi) fuel behind the valve to spray out. The nozzle of the injector is designed to atomize the fuel for 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 then continuously updates injector opening times in relation to the information given by these sensors.

The throttle body is also equipped with an idle air control valve. When the valve opens it allows air to bypass the throttle, which provides the additional air required to idle at elevated speed when the engine is cold. The idle air control motor also compensates for accessory loads and changing engine friction during break-in. The idle speed control valve is controlled by the ECM.

Fuel pressure for the system is provided by an in-tank fuel pump. The pump is a two-stage turbine designed powered by a DC motor. It is designed for smooth, quiet operation, high flow and fast priming. The design of the fuel inlet reduces the possibility of vapor lock under hot fuel conditions. The pump sends fuel forward through the fuel line to a stainless steel high-flow fuel filter mounted on the engine. From the filter the fuel moves to the throttle body. The fuel pump inlet is located in a reservoir in the fuel tank which insures a constant supply of fuel to the pump during hard cornering and on steep inclines. The fuel pump is controlled by a fuel pump relay, which in turn receives its signal from the ECM. A fuel pressure regulator inside the throttle body maintains fuel pressure at 10 psi and routes unused fuel back to the fuel tank through a fuel return line. On the dual throttle body system, a fuel pressure compensator is used on the second throttle body assembly to compensate for a momentary fuel pressure drop between the two units. This constant circulation of fuel through the throttle body prevents component overheating and vapor lock.

The electronic control module (ECM), also called a micro-computer, is the brain of the fuel injection system. After receiving input from various sensing elements in the system the ECM commands the fuel injector, idle air control motor, EST distributor, torque converter clutch and other engine actuators to operate in a pre-programmed manner to improve driveability and fuel economy while controlling emissions. The sensing elements update the computer every tenth of a second for general information and every 12.5 milliseconds for critical emissions and driveability information.

The ECM has limited system diagnostic capability. If certain system malfunctions occur, the diagnostic Check Engine light in the instrument panel will light, alerting the driver to the need for service.

Since both idle speed and mixture are controlled by the ECM on this system, no adjustments are possible or necessary.