A fuel pump is a component in motor vehicles that transfers liquid from the fuel tank to the carburetor or fuel injector of the internal combustion engine.
Carbureted engines often use low pressure mechanical pumps that are mounted outside the fuel tank, whereas fuel injected engines often use electric fuel pumps that are mounted inside the fuel tank (and some fuel injected engines have two fuel pumps: one low pressure/high volume supply pump in the tank and one high pressure/low volume pump on or near the engine).Fuel pressure needs to be within certain specifications for the engine to run correctly. If the fuel pressure is too high, the engine will run rough and rich, not combusting all of the fuel being pumped making the engine inefficient and a pollutant. If the pressure is too low, the engine may run lean, misfire, or stall.
A fuel pump is not necessarily required for the engine to function. The low-pressure fuel needed by a carbureted engine can be supplied simply by mounting the tank higher than the carburetor and allowing fuel to feed with gravity. This method is commonly used in carbureted motorcycles, where the tank is usually directly above the engine, and high-wing aircraft with fuel tanks in the wings.