Heavy Duty Mechanics Practice Test

The main concept here is how fuel metering and timing are controlled. A mechanical fuel pump delivers fuel by a cam-driven plunger with a displacement that’s effectively fixed per engine revolution; the timing is set by mechanical geometry and isn’t adjustable by an electronic signal. In contrast, an electronic unit injector system uses a dedicated injector for each cylinder with electronic control, so the engine control unit can precisely time when injection occurs and how long the injector stays open, plus how much fuel is delivered in each pulse. This per-cylinder electronic control allows optimized spray patterns and timing for different operating conditions, improving efficiency and emissions.

So the key difference is the control method: mechanical timing and fixed flow versus electronically controlled, per-cylinder timing and metering. The other statements mischaracterize the systems—for example, mechanical pumps aren’t electronically controlled, EUI doesn’t rely on fixed flow, and timing control is a hallmark of EUI rather than something it lacks.