Nissan used a big inlet hose to feed the pump with an open screen to keep out the big stuff. If the tank is clean you shouldn't need that filter. The pump will actually pass some pretty big crud and the big filter before the rail will clean it up, under pressure, before the injectors. The inlet filter is over-"engineering", throwing the whole system out of balance.
Sorry this is where you are wrong ZH. Nissan originally used a Roller Vane pump which are relatively insensitive to dirt particles. Roller vane pump will grind dirt up and spit them out.
A Walpro and similar modern pumps are Ceramic Turbine pumps with extremely fine tolerances and they will NOT tolerate any dirt or rust particles getting through. Pre-filters are necessary In fact, Airtex and other companies demand that the fuel tank be removed and the thoroughly cleaned before installation of a new Fuel Pump or the lifetime warranty is null and void.
Here is a Video from Airtex explaining the differences between Roller Van, Gerotor and Ceramic Vane pumps. And the Pro's and Cons of each:
Ceramic Vane Turbines are explained at 4:17.
Note that Ceramic Vane pumps are usually designed as an In-tank pump by most modern manufacturers. The in-tank pump cage usually contains a Pre-filter or sock that attaches to the bottom of the pump. 8:30 in Airtex video.
When used as an External inline pump you must have a pre-filter installed for CV pumps. Nissan did not use any pre-filter on the in tank fuel pipes, because they used an external Roller Vane pump and the Roller Vane pump will tolerate contaminated fuel. AirTex, Aeromotive, Mallory and all electric fuel pump manufacturers recommend some form of pre-filter when using a Ceramic Turbine pump. Either in tank or external.
Edited by Chickenman, 28 June 2016 - 01:45 PM.