Emission Control Systems: Description and Operation

The OSAC system is used on some vehicles to aid in the control of oxides of nitrogen (NOx). The system controls the vacuum to the vacuum advance actuator of the distributor.
A tiny orifice is incorporated in the OSAC valve which delays the change in ported vacuum to the distributor by about 17 seconds (27 seconds on some models) when going from idle to part throttle. When going from part throttle to idle, the change in ported vacuum to the distributor will be instantaneous. The valve will only delay the ported vacuum signal when the ambient temperature is about 60° F. or above. Vacuum is obtained by a vacuum tap just above the throttle plates of the carburetor. This type of tap provides no vacuum at idle, but provides manifold vacuum as soon as the throttle plates are opened slightly. Proper operation of this valve depends on air tight fittings and hoses and on freedom from sticking or plugging due to deposits.