Abstract: Device for injecting a liquid additive including an additive tank connected by an additive pipe to an injector positioned in the exhaust system of the vehicle concerned, where there is provided, on the additive pipe, a positive-displacement lift-and-force pump including a cylinder and a piston which delimits within the cylinder a pressurizing chamber, and where an electric actuator is coupled to the piston to cause it to retreat, drawing additive into the chamber from the tank and simultaneously compressing a spring that acts on the piston, such that by virtue of the spring, the forward movement of the piston, delivering additive to the injector, takes place under controlled pressure.