Abstract: An oscillating or timing valve, sometimes known as an air relay, suitable for use with a chemical injector. A valve housing with sliding poppet, sliding piston and adjustable needle, with the poppet movable in one direction to close an exhaust port and with the piston movable in the opposite direction to engage the poppet and close the outlet or drive port and open the exhaust port. The piston oscillates between the drive port open and the drive port closed conditions, with the timing adjusted by varying the needle orifice and the size of the chamber fed through the needle orifice. The oscillating valve does not utilize any diaphragms or springs.