On any modern heavy-duty diesel (Detroit DD15, Cummins ISX/X15, Volvo D13, PACCAR MX-13, Mack MP8), the fuel filter replacement interval is 25,000 to 30,000 miles in long-haul service, typically tied to the oil change interval. Severe-duty applications (vocational, idle-heavy, dusty environments) cut that to 15,000 to 20,000 miles. The primary fuel/water separator and the secondary spin-on should be replaced together at every interval; replacing one without the other defeats the filtration system's design. Detroit's official spec for the 2014+ DD15, for example, is 25,000 miles for both filters under normal duty cycle, with closer monitoring during cold weather when winter-blend diesel and fuel-station quality can shorten filter life dramatically. The practical rule from fleet maintenance: inspect the primary filter housing for water at every PM stop, replace whenever the sediment bowl shows water beyond the drain mark, regardless of mileage. Run any Fleetguard, Donaldson, or OEM part number through the Steinberg cross-reference search for the current Steinberg SKU. All ship same-day from our Hodgkins, Illinois warehouse for orders before 2 PM CT, NET-30 for qualified fleet accounts.
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