Throttle Body Injection (TBI)

Injector is located in throttle body unit. Dual injectors are used on 4.3L, 5.0L (VIN E) and 5.7L (VIN 7) models. Battery voltage is supplied to the injector when the ignition is on. ECM energizes solenoid by providing a ground path through its internal circuitry. By regulating the injector ground circuit, ECM controls injector "on" time (pulse width) to provide proper amount of fuel to engine.

Pressure to injector is maintained at a constant level by the pressure regulator. Excess fuel passes through pressure regulator and is returned to fuel tank.

In the "run" mode, ECM uses tach (RPM) signal to determine when to pulse injector. Fuel injectors are pulsed once for each engine revolution, each spray providing 1/2 the fuel required for the combustion process. Thus, 2 injections of fuel (2 rotations of crankshaft) are mixed with incoming air to produce the fuel charge for each combustion cycle. On models equipped with dual injectors in the throttle body, injectors are pulsed alternately.

During starting, clear flood mode, deceleration and heavy acceleration, fuel delivery is controlled by internal ECM calibration.

• Starting - During engine starts, ECM delivers one injector pulse for each distributor reference pulse received (synchronized mode). Injector pulse width is based upon coolant temperature and throttle position. Air/fuel ratio is determined by ECM when throttle position is less than 80 percent open. Engine starting air/fuel ratio ranges from 1.5:1 at -33°F (-36°C) to 14.7:1 at 201°F (94°C). At lower coolant temperatures, injector pulse width is longer (richer air/fuel mixture ratio). When coolant temperature is high, injector pulse width becomes shorter (leaner air/fuel ratio).
• Clear Flood - If engine is flooded, driver must depress accelerator pedal to Wide Open Throttle (WOT) position. At this position, ECM adjusts injector pulse width equal to an air/fuel ratio of 20:1. This air/fuel ratio will be maintained as long as throttle remains in wide open position and engine speed is less than 600 RPM. If throttle position becomes less than 80 percent open and/or engine speed exceeds 600 RPM, ECM changes injector pulse width to width used during engine starting (based upon coolant temperature and manifold vacuum).
• Heavy Acceleration - Fuel enrichment during heavy acceleration is provided by ECM. Sudden opening of throttle valve causes rapid increase in MAP signal. Pulse width is directly related to MAP, throttle position and coolant temperature. Higher MAP signal and wider throttle angles give wider injector pulse width (richer mixture). During enrichment, injector pulses are non-synchronized (not in proportion to distributor reference signals). Any reduction in throttle angle cancels fuel enrichment.
• Deceleration - During normal deceleration, fuel output is reduced. This reduction in available fuel serves to remove residual fuel from intake manifold. During sudden deceleration, when MAP, throttle position and engine speed are reduced to preset levels, fuel flow is cut off completely. This deceleration fuel cut-off overrides normal deceleration mode. During either deceleration mode, injector pulses are not in proportion to distributor reference signals.