My city public works trucks are both 2008 Fords with the 6.4. Both were purchased new and both have around 70k miles on them. Earlier this year the F550 developed a low power output issue and threw codes for a low fuel pressure problem. A friend of mine is a Ford mechanic and recommended I replace the fuel filters 2 times but bet it was an injector pump. I replaced the filter and it ran OK for a bit but did it again. We took it to a shop and they replaced the injector pump at the cost of $4,500.