Fuel transfer issue on 04 670.

There should be a shut off valve on both tanks. One may have been closed which would not allow the fuel to transfer from one tank to the other.

 

First I'd Ck the vents on the top of the tanks a clogged vent most of the time is the problem if not
Change both draw fuel lines from the tanks to the T when u replace them thy have to b the same length or u will have the same problem same goes for the returne lines
Hope this help

 

Similar problem here, driver side tank will draw down to almost empty and passenger tank will not draw lower than 7 inches of fuel (24 inch tanks). I put in 70 gallons and figure 7 inches on other side for a total of 110 gallons on board -- good for 600 miles to empty, 375 miles later fuel gauge red light comes on showing dog empty. Measure again and still 7 inches on passenger side, drivers side about 3 inches.

 

They should be on the line coming out of the tank. You might also have a pin hole in the line.

 

If the vents check out OK, then it is the fuel proportioning valve that returns fuel to the tanks. They can go bad, and if they send the majority or returned fuel to one side you will have that problem,

 

I had this same problem on a PornStar. I finally caught the gauge (sender is on the driver's side tank) rising from 1/2 to Full by itself, driving down the road one day. I read somewhere that these newer ones treat the left tank as a reserve, pumping left to right as necessary. If the transfer connections aren't connected in the right order it could be pumping right to left instead, resulting in such a problem.