QUESTION: I’m hoping you can bring an old-timer up to speed on fuel injection. I used to be pretty good at fixing things but hadn’t even considered trying to work on this one. I’d feel better driving if I had an idea what’s going on under the hood.

—Herb Frost

ANSWER: OK, here goes: A typical fuel-injection system utilizes three general processes: fuel pressure, thinking/processing and fuel delivery. Fuel pressure involves the use of an electric fuel pump inside or near the fuel tank. The pump sends fuel to the fuel rail, a pipe of sorts that’s attached to the engine and the fuel injectors. There, a pressure regulator holds a specified pressure and returns unneeded fuel to the tank. Recent vehicles house the regulator in the fuel tank and send forward only the fuel that’s needed.

The thinking/processing part employs a control unit typically called an electronic control module/powertrain control module, or ECM/PCM. In addition to managing the fuel pump, this very smart box receives information from a dozen or more engine and transmission sensors. This information indicates engine and vehicle speed, load, temperature, airflow, throttle position, altitude and gear selection, just to name a few. Incoming data is crunched with programmed instructions resulting in appropriate commands being issued to fuel, ignition, transmission and emission-control actuators.

Fuel injectors manage the delivery of metered, misted fuel, which next vaporizes in the intake manifold on the way to the engine’s combustion chambers. Each brief electrical pulse sent from the ECM/PCM to these electromechanical showerheads results in a precise shot of fuel just as each intake valve opens. Very recent vehicles take this a step further and shoot highly pressurized fuel directly into each combustion chamber. Fuel injection is vastly superior to carburetors of the past. Now, each engine cylinder receives a more evenly matched quantity of fuel, fuel is more precisely atomized (misted), and there are far fewer quirky mechanical things to go awry.

The thinking/processing part now comes back into play, monitoring exhaust oxygen content. This information allows fine-tuning and corrections to be made to fuel-injection commands, in compensation for minor problems or wear and tear. If a problem develops that could threaten emission compliance, and to a lesser degree engine performance, the onboard diagnostics system lights the "check engine" lamp, stores a trouble code, and records a snapshot of engine conditions.

Fuel-injection systems are incredibly reliable. When service is needed, a scan tool provides access to trouble codes, sensor data, fuel trim or fine-tuning information, actuator commands, the data snapshot and additional diagnostic information. Fuel pressure may also be checked by connecting a pressure gauge. Typical problems, while very rare, include a worn-out fuel pump, with a lifespan perhaps of 100,000-150,000 miles; dirty fuel injectors; and sensor or connection faults. Most are fairly easy to diagnose and repair.

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