Diesels have a fuel water separator because diesel fuel naturally absorbs and carries water, and water destroys diesel injection systems faster than almost any other contaminant. Water enters the fuel through three main paths: condensation inside fuel storage tanks, contaminated fuel-station storage, and biological growth (microbes that consume diesel and release water as a byproduct). Once in the fuel, water passes through the high-pressure injection pump and injectors at 30,000+ PSI, causing cavitation and tip erosion that ruins injectors in tens of operating hours. A single contaminated fuel event without a separator can require replacing all six injectors. The separator's job is to coalesce the suspended water droplets into a drainable pool before they reach the injection system. Heavy-duty diesels (Cummins, Detroit, Volvo, Mack, PACCAR) all run a separator as the primary fuel filter for this reason.
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