Why are there two separate drivers for the fuel injectors in a 1992 Chevrolet?

Leaking fuel injectors can cause hard starting when the engine is hot, a lumpy idle, or high emissions (unburned hydrocarbons). TBI engines (non-turbo 2.2 and 2.5, some V8s) have a single fuel injector. Some four cylinder engines used in Mexico had multiple-port fuel injection (MPI; 4 injectors), a far better system for economy, power, and idling quality.

Multiple point fuel injection was phased in starting with the turbocharged engines in the 1980s; the Neon arrived in late 1993 with sequential multiple-point fuel injection, a key reason why the new 2.0 liter engine outperformed the older 2.2 liter engine. The logic module controls the fuel injectors and the amount of fuel supplied to the engine. Single-point injection is used in Throttle Body Injection (TBI) applications.

This uses a single large injector mounted in the center of the bore of the throttle body. This injector has a spray pattern of 6 orifices which spray fuel into the air flow. This flow is deflected off a nozzle to deliver a 45 degree spray pattern.

Similar to a carburetor, the throttle body provides air and fuel through a bore where the butterfly valve regulates the amount of air flowing into the manifold. This butterfly valve is moved using a throttle cable. As the engine speed increases, the logic module cycles the injector on and off at the proper rate to provide the right amount of fuel to the engine.

As the engine speed increases, the logic module provides more fuel into the air flow. (At idle, airflow is controlled by the idle speed motor). For turbocharged engines and other engines which use a multi-port injection (MPI), there is an injector for each cylinder.

The requirement of the injector is reduced because it doesn’t have to supply fuel to the entire engine, so the MPI injectors are smaller but have a similar spray pattern. Since in MPI engines there is one injector per cylinder, the injector sprays fuel directly into the cylinder’s air stream, through the manifold port connected to the intake port of the head; more fuel should stay in suspension this way. Engines controlled with logic modules other than the SBEC use a batch-fired injector.

This means that the injectors are fired in pairs rather than one at a time (sequential). In batch-fired systems, fuel is injected behind the intake valve on the power stroke. This fuel is pulled into the cylinder on the intake stroke as additional fuel is injected during this intake stroke.

The total amount of fuel injected on both sprays ends up being correct, as calculated by the logic module. This system cannot react as quickly or precisely, and some fuel comes out of suspension, but it is nearly as effective. The auto shutdown (ASD) relay supplies 12 volts DC to the fuel injectors.

To activate the spray, the power module provides a ground path, based on a signal from the logic module. The length of this signal is controlled by the logic module on 1984 models, and uses the fuel injector sync signal and ignition reference signal from the Hall Effect Pickup (HEP) to determine which injector should fire. In later models, the power module uses the logic module for the signals to determine when and which injector to fire.

As the logic module turns an injector on or off, it checks the output of the injector driver circuit to ensure that it is working properly.