When changing the fuel filter, if the fuel in the filter bowl looks gray or black, it may be a sign that the fuel injector O-rings are allowing high pressure oil to migrate into the fuel return system. Symptoms of this are rough idle or hard starting.
Oil in fuel system due to injector seal leaks on Ford Excursion
Average mileage: (–)
Engine affected: 7.3L V8 Diesel Turbo
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