Fuel Filter Functions

A car's fuel filter is located along the fuel lines, either in the engine compartment or underneath the car by the fuel tank. It is the responsibility of the fuel filter to trap large particles in the fuel in order to prevent them from getting into the engine. Because of the tremendous force behind the up-and-down motion of the engine's pistons, which compress the air-fuel mixture so that it will burn more efficiently, any large particles in the fuel could potentially cause serious damage to the engine. Large particles in the fuel also have the ability to clog fuel injectors, depriving a cylinder of precious fuel and requiring that the injector be cleaned or replaced. Therefore, maintaining a clean fuel filter is imperative to the performance of a car's engine.
Due to the differences in fuel delivery systems, each fuel filter is different. A carbureted engine, which essentially uses the principle of vacuum to suck fuel into the engine, has a fairly low-pressure fuel system. Fuel in a carbureted system passes through fuel-resistant rubber hoses. The fuel filter is made of metal or plastic, with an inlet tube protruding from one end and an outlet tube protruding from the other; a hose is fastened over each end with a circular clamp.
In a car with electronic fuel injection, the injectors squirt fuel into each cylinder. Fuel in this type of system is kept highly pressurized with the help of a pressure regulator. Due to the high-pressure system, the fuel lines must be made of metal. Sometimes the fuel filter in a high-pressure fuel injected system is placed in a low-pressure section, and may be connected to rubber hoses with hose clamps, much like in a carbureted system. However, the fuel filter in a high-pressure fuel injected system is usually equipped with a threaded fitting on each end that screws into the fuel lines on either side. 
When changing the fuel filter in your car, there are a couple of precautions you need to take. First, in a car with a high-pressure system, you will need to relieve the pressure before disconnecting the fuel lines from the filter. In most cars, this can be accomplished by removing the fuse that controls the fuel pump, and idling the car until it runs out of gas. Another method that works for some cars is to remove the gas cap. Check your shop manual for more specific instructions. 
In order to be sure the fuel filter works correctly, you will also need to take care to place the fuel filter so that the flow travels in the right direction. Most fuel filters mark one side as "in" and the other as "out." The inlet should connect to the fuel lines that come from the fuel tank underneath the back of the car, while the outlet should connect to the fuel lines that can be traced to the engine. Sometimes the fuel filter will have a different kind of fitting on each side, so that it can only be installed in one direction.
Because the fuel filter is one of the key components in protecting the engine from hazardous foreign particles, it is important to replace it regularly. Some mechanics recommend replacing the fuel filter every year during the regularly scheduled tune up, but other mechanics disagree. To be on the safe side, you should replace your fuel filter at least once every two years. Your fuel filter may need more frequent attention if you live in a particularly high-pollution area, or if you put more miles on your car in a year than the average person.