Because the fuel tank is located on the opposite end of the car from the engine, a fuel pump is required to draw the gas toward the engine. There are two kinds: mechanical, which were used in carbureted cars, and electric, which is used in cars with electronic fuel injection.
A carburetor is a fuel delivery mechanism that makes use of the simple principle of vacuum in order to deliver fuel to the engine. The same vacuum that draws the air-fuel mixture into the engine also draws fuel along the lines toward the engine. However, additional help is necessary, so carbureted engines have a mechanical fuel pump. This runs off of the engine's rotation; as a result, in a carbureted car, it is located alongside the engine.
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Electronic fuel injection is a delivery system that squirts a fine mist of fuel into the combustion chambers of the engine. A computer controls the system, closely monitoring factors such as the position of the throttle, the air-fuel ratio, and the contents of the exhaust. Because the system does not use a pre-existing force, such as vacuum, to draw the fuel along the lines, the fuel pump must be located at the source — that is, inside or next to the fuel tank itself. The pump is electronic, meaning that it is powered and controlled electronically. Sometimes, its operation can be identified by a soft, steady humming sound coming from the rear of the car.
Fuel pump failure is not uncommon, particularly in cars with electronic fuel injection. Usually, when it fails, a car will simply sputter and die, and will not restart. Essentially, a car with this failure will act like it is out of gas, even when there is gas in the tank. Fuel pump failure can be verified by checking the fuel delivery end of the system; if no fuel is being delivered to the engine, the fuel pump has most likely failed.
Replacing an electronic fuel pump can be tricky business. In some cars, it is located in an area that is easy to access from underneath the car. Other cars have an access panel in the interior of the car that can be removed to reach the fuel pump. Still other cars require the fuel tank to be siphoned and removed, or dropped, before the pump can be accessed. The latter type of car usually makes for the most laborious replacement job.