Fast Idle Deceleration System: Operation: California
One circuit of system pulls vacuum at throttle opener port, the other at intake manifold. When the throttle plate closes, creating more than 10.6 in. (270 mm) Hg, the check valve closes and the throttle plate opener is pulled open. Vacuum drops slowly as it is bled off by the delay valve.
When vacuum drops too low, the check valve opens. Intake manifold vacuum is applied and controlled by vacuum regulator at 6 in. (150 mm) Hg. This vacuum keeps the throttle plate positioner half-open. When vehicle speed drops below 17 mph, the speed sensor turns off the solenoid valve. Manifold vacuum is cut off and the throttle closes.
A separate dashpot (not vacuum controlled) keeps the throttle from closing completely for 2-4 seconds.