Fuel Pump

Almost all modern cars have an electric pump in the tank. It runs whenever the engine runs and briefly when you switch on the ignition. Direct-injection petrol engines and diesels add a mechanical high-pressure pump driven by the engine. Many cars control the pump speed according to demand. A check valve holds pressure in the line after the engine stops, which helps hot starting.

Most cars have an electric pump in the tank. Direct-injection petrol and common rail diesel engines also have a mechanically driven high-pressure pump on the engine.

The in-tank pump pushes fuel through the filter to the engine. A regulator or the control unit keeps pressure within a set range; a pressure sensor reports the actual value on many systems. Many modern pumps vary their speed according to demand, controlled by a separate module. A pressure sensor tells the control unit whether the requested pressure is reached.

A failing pump causes hard starting, stalling, hesitation under load and low fuel pressure codes. Running on an empty tank regularly can shorten pump life, because fuel helps cool it. A weak pump causes hesitation under load, hard starting or stalling. Regularly running the tank almost empty can make the in-tank pump run hotter, because fuel helps cool it.