A fluid injector, for example for injecting liquid fuel into a combustion chamber, comprises a valve housing and an injector assembly which may be removably mounted on the valve housing. The injector assembly includes discharging means for discharging fluid from the injector assembly, ducting for conveying the fluid between the valve housing and the discharging means and a control valve movable to a first position in response to flow of the fluid in a first direction through the ducting to close the discharging means and return the fluid from the discharging means to the valve housing and movable to a second position in response to flow of the fluid in a second direction through the ducting to open the discharging means to allow at least some of the fluid conveyed to the discharging means to be discharged therefrom and to return any surplus fluid to the valve housing. Provided in the valve housing are inlet and outlet passages for conveying the fluid into and from the valve housing, and a change-over valve. The change-over valve comprises a valve member adjustable into a first position to condition the injector for flow of the fluid from the inlet passage through the ducting in said first direction and into a second position to condition the injector for flow of the fluid from the inlet passage through the ducting in said second direction. In the valve housing there is also provided a spill return valve for varying the proportion between the amount of the fluid that is allowed to discharge from the discharging means and the amount that is returned from the discharging means to the valve housing when the control valve is in its second position.
