Fuel and Air Intake
The typical fuel filter for most fuel-injected cars consists of a high-pressure canister filled with filtering media. Filters may have clamped, threaded or special fittings to ensure reliable connection to the fuel system. Filters for carbureted engines may be located at the inlet of the carburetor or inline. Filters for carbureted engines do not need to withstand the same pressures as those for fuel-injected engines.
Fuel filters trap harmful contaminants that may cause problems with carburetors and intricate fuel injectors. Fuel filters for carbureted engines only clean the fuel before it enters the float bowl. Injection filters, on the other hand, clean the fuel whenever the fuel pump runs (unless the fuel injection system is a "returnless" design). Fuel moves continuously up the supply side, through the filter to the fuel rail or throttle body. The fuel that doesn't make it into the engine returns to the tank and the whole process starts over again. With a full tank of gas, the filter may clean the volume of fuel in the tank many times before it's all used.
Since "normal" usually constitutes severe driving because of less than normal conditions, it's best to replace the filter every two years or 40,000 kilometres. A contaminated filter can restrict fuel flow from your car's electric fuel pump, eventually taking a toll on its life. Frequent filter replacements remove all doubt about whether the filter may cause other problems down the road.
Because of these technicalities and because of most filter locations, it's best to have your car's fuel filter replaced by a qualified service professional.