It's likely the diaphragm in the vac solenoid is split - similar fault that occurs with the secondary throttle vac actuator. My bet will be this vac solenoid is normally hooked up to a thermal switch like the unit you'd find screwed into the base of the thermostat housing in the inlet manifold (got a feeling this unit operates the fast idle function). Once the thermal switch reaches operating temp it will allow a vacuum circuit to actuate the solenoid and disable the fast idle function (I'm taking a stab at this but I could be wrong...)