By Larry Carley, Copyright 2007
An electric fuel pump is used on engines with fuel injection to pump fuel from the gas tank to the injectors. The pump must deliver the fuel under high pressure (typically 30 to 85 psi depending on the application) so the injectors can spray the fuel into the engine. Fuel pressure must be within specifications for the engine to run correctly. Too little pressure can starve the engine for fuel, causing it to run lean, misfire, hesitate or stall. Too much fuel pressure can cause the engine to run rough, waste fuel and pollute.
Electric fuel pumps are usually mounted inside the fuel tank, though some may be mounted outside the tank. Some vehicles may even have two fuel pumps (a transfer pump inside the tank, and a main fuel pump outside). The in-tank location helps muffle the buzzing noise produced by the electric pump motor, and immersing the pump in fuel helps lubricate and cool the pump motor. Driving with the fuel tank less than 1/4 full can shorten pump life by causing it to run hot. It also increases the risk of momentarily starving the pump for fuel when cornering sharply, braking or accelerating. Running out of gas can sometimes damage an electric fuel pump by starving it for cooling and lubrication.
Electric fuel pumps come in a variety of designs: single or double-vane, roller vane, turbine or gerotor style pump. The pump runs continuously as long as the engine is running, and does not stop until the engine is shut off.
The pump is usually part of the sending unit assembly, that includes a float that sends an electrical signal to the fuel gauge on the instrument panel. If the fuel pump needs to be replaced, it can be replaced as a separate item or as a complete assembly (which is more expensive but easier and less troublesome).
An electric fuel pump is powered by battery voltage routed through a voltage relay. The fuel pump circuit has a fuse for overload protection, and may also have an inertia safety switch that shuts the fuel pump off in the event of an accident. The voltage supply circuit may also sense engine oil pressure or engine rpm, and cut off power to the pump if the engine has stalled or is not running (possibly as the result of an accident or broken fuel line).
The fuel pump should last the life of the vehicle, but it may fail as a result of contamination inside the fuel tank (dirt or rust), fuel starvation (running out of gas), overheating (always driving with a low fuel level), low voltage (wiring problem), or overwork (trying to overcome a restriction caused by a plugged fuel filter). The harder a pump works, the hotter it runs and the more amps it pulls through its power circuit.
A fuel pump can be expensive to replace. A new electric fuel pump may cost from $100 to $300 or more depending on the application, and whether you are buying just the pump or the complete fuel pump sending unit assembly. The labor to replace a tank-mounted pump can also add $200 or more to the repair bill. So you want to make sure a bad fuel pump is the real problem and not something else before you replace the pump.
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