The key idea is that rail pressure is controlled by a vacuum-sensing regulator on the fuel rail. This regulator has a diaphragm that compares pump pressure on one side with the intake manifold vacuum on the other, often aided by a spring. When the engine is idling and manifold vacuum is high, the regulator pulls the diaphragm toward the engine, restricting the return flow and lowering the rail pressure to a set level below the pump pressure. As engine load increases and manifold vacuum drops, the diaphragm is pushed by the spring, allowing more fuel to bleed off back to the tank and letting rail pressure rise toward the pump pressure. This keeps the pressure differential across the injectors within a narrow, appropriate range so the engine gets the right amount of fuel for any speed or load. Injectors themselves are only the spray devices controlled electronically; the flow divider merely distributes fuel; the fuel pump supplies pressure but doesn’t adjust it based on manifold vacuum.

During idle, the engine relies on precise fuel metering to keep the air–fuel ratio steady so the engine runs smoothly. If the metering starts to fail, the air–fuel ratio won’t stay constant, causing the engine to stumble and misfire as some cylinders get too much or too little fuel. That combination—rough idle with fluctuations in AFR and potential misfires—is a clear sign of a fuel metering issue at idle. A smooth idle with a steady AFR means the metering is working correctly. No exhaust or overheating aren’t direct indicators of a metering problem at idle.

The lever position control input is what the engine control uses to decide how much fuel to meter. To do that accurately, the electronic engine control needs a direct, continuous measure of how far the power lever is moved. The power lever angle potentiometer sits on the power lever and turns the lever’s angle into an electrical signal that the EEC can read. This signal tells the control exactly how much thrust is being requested, so it can adjust fuel flow accordingly. The other devices have different roles: a hydromechanical fuel control is the older metering mechanism driven by hydraulic and mechanical inputs rather than a direct electronic lever-sensing signal; the electronic fuel control unit is the computer that processes inputs including the lever angle signal; and a venturi measures air flow, not lever position. So the sensor that senses the power lever angle is the power lever angle potentiometer.

Spray strategy is about where and how fuel is atomized and introduced into the engine. In direct injection, fuel is delivered at high pressure and injected directly into the combustion chamber, allowing precise metering and rapid vaporization inside the cylinder. In port injection, fuel is sprayed from injectors in the intake port at a lower pressure, where it mixes with incoming air in the intake manifold before entering the cylinder. That distinction is exactly captured by the statement: direct injection meters fuel at high pressure directly into the cylinder, while port injection sprays fuel at lower pressure into the intake port. The other descriptions mix up the spray locations or claim the systems are identical, which isn’t accurate.