A fuel/water separator typically lasts 25,000 to 50,000 miles on a heavy-duty diesel engine, replaced at the same interval as the primary fuel filter. Severe-duty applications, cold weather operation, and routine fueling at lower-grade fuel stations all shorten that interval. The sediment bowl needs draining far more often than the filter element needs replacement: drain at every fuel stop, swap the element at OEM mileage interval. The element's service life depends on water content and particulate load in the fuel. Heavy water contamination can saturate a coalescing element in a few thousand miles. Heavy particulate load can clog the filter media before water exhausts the coalescer. Inspect the housing pressure differential at every PM stop; a pressure drop above spec means the filter is restricting flow, regardless of mileage.
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