Mixture Control System: Operation

On 1974 mixture control system, when engine is at idle or running at low speeds, the speed sensor signals computer to cut off current to vacuum switching valve. No vacuum is allowed to reach mixture control valve. During intermediate and high speeds, vacuum switching valve permits vacuum to reach mixture control valve diaphragm. Vacuum, however, is not high enough to activate valve. When throttle is released suddenly, additional vacuum is created, causing mixture control valve open momentarily and allow fresh air to be drawn into intake manifold.

On 1977-79 system, when sudden deceleration occurs, high manifold vacuum acts on the lower chamber ("B") of the mixture control valve. The valve opens and fresh air is drawn in through lower side of valve, through filter. This air is transmitted to the intake manifold where it helps maintain a balanced air/fuel mixture.

After a few seconds, the vacuum in the upper chamber ("A") will balance vacuum in the lower chamber. The mixture control valve closes and no additional air is brought into the intake manifold (deceleration fuel control is then maintained by the throttle positioner or deceleration fuel cut system).