The Detroit DD15 has two fuel filters in series: a primary filter (also called the pre-filter or fuel/water separator) mounted off-engine, and a secondary filter (the spin-on fine filter) mounted on the engine itself. Both work together to protect the high-pressure common-rail injection system. The primary's job is to drop free water and large particulates into a sediment bowl; the secondary catches everything finer that the primary missed before the fuel reaches the injectors. Both must be serviced together at every interval; replacing one and not the other defeats the system's design. On a 2014+ DD15 in long-haul service, the primary is typically Fleetguard FF5613, FF5613B, or FS19975 depending on configuration, and the secondary is FF5612 or FK13245 for the kit version. Steinberg's cross-reference for the DD platform ships same-day from Hodgkins, Illinois, NET-30 for qualified fleet accounts.
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