Help: I have an F150 2 whl drive pickup, manual transmission. Straight 6 cylinder.
The gas flows from front tank into the rear tank. Not a huge problem, but when you fill both tanks, gas starts running out of the rear tank because the front one is still trying to fill the rear one. Gas mileage doesn't look good at all, when it is like this. I have changed all of the plugs and filters, where do I go from here?

It's a pretty expensive part , can't rember the exact name for it but it's a valve under the truck that switches between the two tanks, everyone I know w/ that truck has the same problem, all the 90 model fords w/ dual tanks

OK I just had the same thing go wrong w/ my truck . I replaced one fuel pump and it started pumpin gas in to the back tank,
I replaced the rear pump and now my problem is fixed.
It is an easy fix I changed both pumps by myself in half a day, and I dd it in my front yard shade tree style