Just got back from a 2800 mile round trip to Wyoming. Pulled a 36 Foot 5th wheel camper with a 2018 Ford 250 diesel. I have a auxiliary tank which is gravity fed and I was hoping that it would keep up with the 9 mpg but it didn't. I still had to make several stops on the way with an almost full auxiliary tank. It keeps up fine around town when i am getting 13 or 14 mpg. Is there something i am missing or do I need to install a pump.

 

Is it vented??

 

yes it is vented

 

Well a free flowing 1/4" line will flow a gallon or so per minute. I'm sure your not doing over 9 miles a minute (540 mph) so, you have some sort of a restriction, probably dealing with the vent. You can either enlarge the vent pathway or put in a valve and pump.

 

Check your vent hose for debris. I had a 60 gallon aux tank on my 6.7 and it kept up flawlessly. Open it up at half a tank and my fuel needle would go up. I ended up with a mud dauber wasp making a nest in my vent tube. Cleaned it out and ziptied a piece of window screen to the end of the vent tube and it never happened again.

 

just checked the vent and its fine.

 

I'm gonna disconnect the hose and time it. So i should get 1 gpm?

 

Well, the theory of gravity is supported everyday with the workings of the universe, I really doubt your fuel tank will disprove what we know of gravity. Check air going into the tank and the pathway of the fuel into the main tank. Something is impeding the flow of air or fuel. I would remove the tank vent/lid entirely and check your flow and go from there.

 

The vents have tip over valves in them too. A little ball that plugs the vent up in the event of a roll over. Might be worth checking. There's also an over flow check valve where you spliced into the fill tube, it may be jammed up too.