Abstract: An improved fuel injector (10) having a needle valve (31) is mounted within a separate removable guide (26) for sliding movement between seated and unseated positions on a valve seat (18). Fuel injector (10) is adapted for operation in a primary fluid control mode (FIG. 5) under the influence of pressurized control fluid in a fluid chamber (30) adjacent the outer end of the needle valve (31), and for alternate operation in a secondary mechanical control mode (FIG. 6) under a mechanical mode spring (96) in the event of a decrease in operating fluid control pressure of the pressurized control fluid below a predetermined minimal amount. Continuous operation of the fuel injector (10) in the alternate operation under the mechanical spring (96) mode is automatic upon the reaching of a predetermined minimum pressure in the operating control fluid. A shutdown system is illustrated in FIG.