Fuel injector with hydraulic coupling unit

The fuel injector comprises a hydraulic coupler unit (20) having a coupler piston (22) and a valve piston (23), where the coupler piston is pre-stressed by a pretension spring (40). The coupler unit is arranged in a low pressure space (12), where the coupler unit comprises a compressed reservoir (50) which is formed at the coupler piston. The compressed reservoir is limited with a flexible wall (51) for the low pressure space.