A dual-mode fuel injector has an internal part containing a variable volume chamber space which fills with fuel at common rail pressure and a plunger which is operable to force fuel out of the variable volume chamber space and to be injected out of the nozzle through the injection orifices at amplified pressure greater than common rail pressure. The plunger and the internal part have respective surfaces which come into mutual abutment during filling of the variable volume chamber space with fuel at common rail pressure and which, when in mutual abutment, create a seal which seals fuel in the variable volume chamber space against escape from the variable volume chamber space past the seal.