Fuel Shutoff on Decel (Non-Turbo Only)

Fig. 23 Fuel Shutoff System:

Manual transmission equipped vehicles are equipped with a system, Fig. 23, that shuts off fuel injection during engine overrun. The system is provided to lower the amount of emissions entering the atmosphere and to lower the fuel consumption. The system is comprised of a vacuum valve and a decel valve.
The decel system causes the induction air that normally flows across the air flow sensor plate to bypass the sensor plate via a hose and vacuum valve. This results in the sensor plate remaining in its home position, thereby shutting off the flow of fuel to the cylinders. Thus, the vacuum valve acts as a bypass line when the decel system is functioning. Because the relay is operating, the ECU selects a fixed pulse ratio of 60%. The decel relay will only operate when the thermostatic switch is closed (coolant temp. above 113°F (45°C); the time relay (P11) is not operating (relay in de-energized position); the engine speed is higher than 1400-1750 RPM; and the throttle position sensor is making the circuit.
The decel system shuts off the supply of fuel as soon as the set of contacts in the throttle position sensor close (foot removed from the accelerator). Fuel supply returns once the engine speed has dropped below 1300-1450 RPM.

Fig. 24 Fuel Shutoff System Schemetic:

When current flows through the vacuum valve solenoid, Fig. 24, from the power distribution panel via the decel relay to ground, the depression in the inlet manifold acts on the diaphragm in the valve, opening the bypass port.