The Ford Focus mixes fuel and air under pressure with a fuel injector. Carburetors previously performed this function passively but were rendered obsolete by fuel injectors in the late 1980s. A fuel injector's programmable firmware allows it to handle different fuels. A fuel injector is comprised of only the nozzle and valve; it still requires the pump to pressurize the fuel.
Determine which fuel injector your Ford Focus will need. This depends not only on the make and model, but also the engine size. Models made from 2000 to 2007 can be equipped with a 2.0L or 2.3L engine. Models made after 2007 will have a 2.0L engine.
Relieve the fuel system pressure and disconnect the negative battery cable with a socket wrench. Remove the electrical connector and vacuum hose from the fuel injector.
Release the retaining clips from the fuel injection supply manifold and detach the fuel supply line. Loosen the mounting bolts for the fuel injection supply manifold so you can take out the manifold. Remove the fuel injector and discard the old O-rings.
Install the new fuel injectors into the fuel injection supply manifold and apply a small amount of clean engine oil to the new O-rings before installing them. Use a torque wrench to tighten the mounting bolts for the fuel supply manifold to 18 foot pounds.
Attach the fuel supply line, vacuum hose, and electrical connector to the fuel injector. Connect the negative battery cable with a socket wrench.
Start the engine and check for fuel leaks.