Fuel filters are used to strain contaminates from the fuel before it reaches the fuel injectors. Keeping the injectors clean is important in order for them to function properly.
On traditional EFI systems there is a pre-filter just before the inlet to the fuel pump. This filter is used to catch the larger contaminates and protect the fuel pump from damage. Think of it like the filter sock used on the end of an in-tank carbureted fuel pickup. It filters out the large contaminates.
Before the fuel reaches the fuel rail it needs to be filtered again to remove the smaller contaminates. That job is done by a filter that has a finer filtering media to catch the smaller contaminates. A popular OE filter used on late 1990’s GM pickup trucks is shown in Photo 1. It is inexpensive, very effective, and available at just about every auto parts store in the USA.
Photo 1. A late 1990’s GM truck in-line fuel filter.
On many modern cars, including the new 5th-generation Camaro, Cadillac CTS-V, etc, there is only one fuel filter. It is located in the fuel module and is attached directly to the fuel pump. In this arrangement flushing the fuel line is necessary before final attachment to the engine fuel rail. It also makes for a very clean installation since no external filters need to be mounted on the chassis or in the engine compartment.