The year was 1985, and Ford truck enthusiasts officially had to face reality: fuel injection was here to stay. Those who'd grown up with jet changes and screwdriver tune-ups might have been a bit leery of these newfangled electronic controls, but Ford quickly proved that it was no rank amateur when it came to designing simple, world-class fuel injection systems.
All of the injectors used over the F-150's electronically controlled history have worked in more or less the same way as any other injectors. These electric injectors are essentially spring-loaded linear actuators -- solenoids -- similar in principle to an electric motor. An wire-wrapped metal ring wrapped around a spring-loaded rod does nothing unless it gets a bit of electricity; when it does, the ring turns into an electromagnet, which pushes up on the rod. At the bottom end of the rod is either a skinny needle, or a fat, tapered cone. When that pointed valve end lifts out of the tiny hole in the bottom of the injector, pressurized fuel shoots out of the hole and into the engine. When the electric supply shuts off, the spring forces the rod downward and the needle or cone blocks the hole again.
A fuel injection computer -- or "injector driver" -- sends power to the injector, telling it when to open. The computer determines the appropriate "pulse width," or how many milliseconds to leave the injector open. As long as fuel pressure remains consistent and fuel isn't restricted by dirt or buildup in the injector, opening the valve for a given number of seconds will always introduce the same amount of fuel. But the injectors can only stay open for so long; if they're not given enough time to "rest" between openings, the electricity in the coil will burn the injector up. Most injectors like those used in the Ford 5.0-liter can run at about 75 to 80 percent "duty cycle" without burning out.