Cummins recommends replacing the primary fuel filter at every oil change interval, which, on a Cummins ISX, ISX15, or X15 in standard long-haul service is 30,000 to 50,000 miles depending on duty cycle and oil specification. The secondary water-separator typically gets the same interval. For ISB6.7 medium-duty, the interval is shorter, in the 15,000 to 20,000 mile range. Severe service (vocational, high-idle, dusty environments) cuts intervals roughly in half. Cold-weather operation can collapse a filter's working life dramatically, sometimes plugging within 3,000 to 4,000 miles when fuel quality drops or wax precipitates out of cold diesel. The practical rule from fleet mechanics: inspect the primary fuel filter every 5,000 miles in winter or after refueling at any station you do not trust, regardless of what the OEM mileage interval says.
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