FRAM Diesel Fuel Filters FRAM Diesel Fuel Filters. The full line of FRAM diesel fuel filters ensures proper fuel system flow while protecting critical system components. As you know, primary filters are designed for optimum balance between low restriction and high efficiency, while secondary filters are Diesel Filters - Fram The purpose of a diesel filter is to protect the injection system on diesel vehicles. Diesel filters eliminate impurities present in the fuel in various ways: Contamination during production, transport, stocking, repairs, etc. Particles entering via the fuel tank ventilation system. Contamination by dirt and oxidation in the fuel tanks or pipes.