See Figure 1Gasoline Engines Only
This system, used in 1979-84, is designed to reduce the engine's oil consumption when the intake manifold vacuum increases to an extremely high level during deceleration. The system consists of two units. A boost control unit as the vacuum sensor, and a by-pass air control unit as an actuator. The boost control unit senses the manifold vacuum. When the level of the manifold vacuum increases above the pre-determined value, the boost control valve opens and transmits the manifold vacuum to the by-pass air control unit. The manifold vacuum then pulls the diaphragm in and opens the by-pass air control valve, thereby causing the air to be bypassed to the intake manifold. After completion of the air by-pass, the manifold vacuum is lowered. This results in the closing of the boost control valve and then the closing of the air control valve. This system operates in a tightly controlled circuit so that the manifold vacuum can be kept very close to the pre-determined value during deceleration.
Aside from a routine check of the hoses and their connections, no service or adjustments should ever be necessary on this system. If at some time you feel that an adjustment is required, it is suggested that you take the truck to a Nissan/Datsun dealer or an authorized service representative.