The electric fuel pump came into wide use on vehicles when fuel injection was introduced. This is due to the increase in fuel pressure necessary for the fuel injectors to work. Conventional engine-driven pumps are unable to produce a high enough pressure for fuel injection. An average fuel injection system requires from 25 to 60 pounds of continuous pressure, depending on manufacturer.
Fuel pumps producing this amount of pressure are fast spinning, loud and tend to heat up if not cooled. For this reason, they were placed in the fuel tank where they are continuously submerged in the fuel for cooling and noise suppression. There is a fuse and a relay for the fuel pump located under the hood of the vehicle in the fuse relay panel. One of the terminals on the relay is hot at all times and the opposing terminal runs to the fuel pump. Without an activating switch, the relay is open.
When the ignition is turned on, there is power to a second terminal, however, the ground to this terminal goes to the computer. For the relay to activate and turn the fuel pump on, the computer must ground the terminal. On start-up, the computer grounds the relay and turns the fuel pump on. However, if the computer does not receive a signal within two seconds that the engine is running, it will shut off the fuel pump for safety reasons. It you sit in the vehicle and cycle the key on and off every five seconds, the fuel pump can be heard to run for only two seconds before it shuts off. It will turn on only when the engine starts.
The fuel pump is usually (on most models) incorporated in a housing with the fuel level sending unit. This is not always the case, but the parts dealer can indicate if it is a separate entity. Having it separate makes it much cheaper to buy just the fuel pump rather than the whole package with the sending unit. The fuel pump in a housing has a anti-rollover valve to prevent fuel from draining from the tank in a rollover. It has a sock or strainer located on the bottom of it to keep sediment out. Generally the whole fuel tank must be removed to gain access to the pump. If the vehicle is a passenger car, before removing the gas tank, look under the mat in the trunk and under the rear seat to see if there is an easy-access panel.
A fuel pump is regulated by a fuel pressure regulator, which is located on the fuel rail that contains the injectors. Its purpose is to keep the fuel pressure at exactly the manufacturer's standards and to allow any excess fuel pumped above that specific pressure to return the fuel to the tank through the smaller pressure return line.
