It is very important to supply clean fuel to the fuel rail in EFI systems. Small particles of dirt can block an injection nozzle and cause an irregular spray pattern.
Any water in the fuel will corrode the inside of the injector - especially if the engine stands unused for long periods.
The first filtering occurs with a strainer or fine gauze in the fuel tank.
The next time filtering occurs at the in-line filter, on the high-pressure side of the pump. This is a large-capacity filter encased, in a steel shell or an aluminum housing. This housing must be rigid, to withstand the high pressures in the system.
The filter is a pleated paper type with pore size of about 10 microns. A fluted support plate keeps the filter stable in the housing. The filter is directional, and it must be fitted in the direction of fuel flow.
Final filtering occurs with a small conical filter at the fuel entry to the injector.