A fuel injection system for an internal combustion engine includes an engine management system, a return conduit connected to a low-pressure fuel system, and a common rail for storing and supplying a relatively high-pressure fuel to injectors. Each injector has a nozzle for injecting fuel into the engine, a valve installed between the common rail and the nozzle, and a spill valve connected by its inlet to the outlet of the valve and by its outlet to the return conduit. A first automatic isolating valve is installed between the common rail and the injector and a second automatic isolating valve is installed between the return conduit and the low-pressure fuel system.