Abstract: A valve is disclosed which is designed for injecting air into an internal combustion engine fuel/air inlet system under controlled conditions, i.e., no air is injected in the engine idle phase (typically under high manifold vacuum from 17 to 20 inches of mercury (in. Hg)). Air injection takes place by means of this valve as a differential function of manifold pressure which is related to throttle opening, (typically starting at about 16 in. of Hg and fully open to atmospheric air at about 8 to 10 inches of Hg, corresponding to open throttle conditions at normal vehicle cruise. The injection valve disclosed herein is designed for installation in series with the engine crankcase scavenging hose on the manifold side of the pollution control valve (PCV). Air flow through the manifold injection valve is controlled by a spring loaded piston and cylinder assembly which is actuated by a vacuum control hose connected to a take off point on the engine inlet manifold.