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First time poster, new to this format. I have an 05 350 crew, 6.0. auto, less then 40K miles. Turbo was serviced in the past and upgraded turbo half installed. Runs fine until I purchased a 5th wheel and towed it home. on long uphill grads it would intermittently start making a fan rev'ing up sound and I would lose speed. Turbo boost look strong @ 20-25 at 3k rpm but once on one of the last times it happed bad, I saw the boost drop to 0 for a short bit before snapping back. Once over the crest of a rise and the throttle let up, the fan sound would usually fade and the drive would engage normally. As it got worse and persisted, I would pull over for a minute and shut down and restart and run strong till another hill load, but mostly the longer hill loads would make it go. I fear the worst.
Kind of sounds like the engine cooling fan engaging as the engine heats up. These powerstrokes have huge fans and you can deffinatly feel and hear it when they fully lock up.
You need to get an OBDII data gauge like the Scanguage II so that you can read your oil and coolant temps. If the temps get too hot it can cause the engine to de-power, but you would normally expect a wrench light to come on. The other common thing that causes a wrench light and lack of power is over boost.
Coolant and oil temps are part of what drive fan operation, and can also cause the engine to defuel. Until you get a gauge and can read actual the actual ECT and EOT numbers, you'll be guessing.