The fuel pump transfers gasoline from the fuel tank to the device that mixes it with air (carburetor or fuel injector) to be ignited and burned in the engine's combustion chamber. The fuel pump in a car is usually electric but can also be a mechanical pump. Carbureted engines use low-pressure mechanical pumps while modern direct injection systems require high-pressure electric fuel pumps.

When the ignition key is switched to "on" the Powertrain Control Module (PCM) energizes the fuel pump's electric motor by turning on a relay. The motor then begins to spin. This rotation generates the measurable fuel system pressure that is displayed on the gauge.

To maintain consistent fuel pressure, the PCM controls the amount of time the fuel pump's motor is allowed to run through pulse-width modulation (PWM). This signal, which is sent from the PCM to a special fuel pump control valve (referred to as the fuel pump controller), is a rapid on/off electrical signal that determines how much the fuel pump rotates.

In some cases, fuel pump problems can cause the vehicle to stall when driven under load. This is a common misdiagnosis and is often the result of a bad fuel filter or incorrect PCM fuel system programming that does not allow for sufficient powertrain demand.

A healthy fuel pump should be able to supply at least twice the vehicle's specified fuel pressure when activated under load. A fuel pump flow test is a quick and inexpensive way to test fuel pump performance. Disconnect the fuel pump feed line from any convenient point and attach a hose that s to a container. Activate the pump for 15 seconds and measure the volume of fuel that flows into the container. A healthy fuel pump should easily be able to supply one pint of gas in this time.

The mechanical fuel injection pump's job is to meter out the correct quantity of diesel fuel for the common rail fuel injection system. This ensures that the fuel injection system has the correct pressure to spray a precise mixture of fuel and air into the combustion chamber. The fuel injection system is a sophisticated electronic control unit that is designed to meet the engine's precise requirements.

The electrical fuel injection pump is a high-pressure fuel pump that meters out the correct quantity of gasoline for the engine's sophisticated electronic fuel injection system. The fuel injection system is a complex electronic control unit that is designed to deliver a precise mix of fuel and air into the engine's combustion chamber. The fuel injection system is essentially a series of electronically controlled nozzles that are used to spray a fine mist of the mixture into each cylinder or throttle body and is capable of spraying up to 60 times per second. The fuel injection system is a sophisticated control unit that is designed to respond to the engine's precise needs to produce power and meet strict emissions standards.