My 2008 f 350 with 46k miles is acting odd. While driving down the road at 65 mph, the turbo gauge wil go from 9 psi to 0 psi, back to 9 or 10, then back to 0 or 1. I get horrible fuel mileage when this happens. I have to drive it 40-60 miles before the turbo acts correctly and stay between 9-10 psi and my fuel mileage goes back up.
Any idea's?
Thanks,
Ken

 

Does the truck sit and idle for any extended periods of time? That's a lot of miles for an 08 already! Does it throw out any codes?

 

No it doesnt sit & idle, no codes are sent.

 

usually when there is little to no boost your mileage will be better. Mine will move a little as well going down the road based upon how hard the motor is working but that small amount should not decrease the mileage.

Not sure what the issue is but would like to know as well.

 

When was the last time you had the PCM calibration updated at your dealer?? Any aftermarket parts?

 

You might also want to slip on down to the 6.4L section here and see if those guys can help you out. Sometimes they don't look in this section.
https://www.ford-trucks.com/forums/forum169/

 

I just bought it & I do not know if the PCM calibration has been updated, but no aftermarket parts.
Thanks,