Mercury - Fuel Filter, 15 Micron Fuel System 35-866171A01. Overview. On MerCruiser engines running the Gen III fuel cooler, this 15-micron filter is one of those parts that quietly keeps things running right. When it starts getting clogged, the engine usually gives you little hints-maybe it falls on its face when you throttle up, maybe cruise speed feels softer than usual, or the motor just doesn't respond the way you expect. Swapping in the proper OEM filter brings the fuel system back in line