Engine Fuel and Fuel Metering Systems Oral Practice Test

The main idea is that the fuel pump relay is a controlled switch for the pump. When the ignition is on, a small control current energizes the relay coil, closing the contacts and delivering battery power to the fuel pump so it can pressurize the rail and supply fuel for starting and running. When the engine isn’t supposed to be running (or ignition is off), the relay opens and isolates the pump, preventing unnecessary battery drain and reducing risk if something goes wrong. The engine control system may also prime the pump or switch it on during certain operating conditions, but the essential purpose is to turn the high-current pump on and off with a low-current signal. This isn’t about directly regulating rail pressure or controlling other systems like exhaust gas recirculation or injector duty cycle; those roles are handled by the fuel pressure regulator and pump, the EGR system, and the injector control logic, respectively.