Deceleration Valve: Description and Operation

Fig. 10 Overrun valve location and exploded view:

These valves were fitted only up to engine number 8S64244.


An overrun valve is fitted to each intake manifold. At high RPM, when throttle is closed, overrun valves open and supply extra air to engine causing lean mixture.


Some 12 cylinder engines are equipped with overrun valves mounted to the front of each intake manifold. The valves are designed to open during periods of high intake vacuum. This process serves to maintain a combustible mixture at all times.