Good afternoon guys! I recently acquired an '88 f150 with a 351 Windsor for a screamin deal (Traded a 99 neon i picked up for 300), but it does have a few nics and tics. Main one being the front tank has a wicked gas leak on the underside of it. My intention is to run off of the rear tank only. When i hit the switch for the rear tank the fuel gauge reads above and beyond full, however I am almost certain that it is still running off the front tank because it'll still run out of gas even though there is gas in the rear.
My goal was to figure out how to bypass that switchable valve, and swap the fuel pumps if finding out the rear pump is bad. But what explains that gauge reading beyond full in the rear tank? The truck gets 8 miles to the gallon plus that gas leak cost me a lot of money at the moment lol. I figured if i have one good tank and all the parts to make it work, i might as well make it work right ?
The sending unit is what controls your fuel gauge. The pumps will swap on that provided you can get them out, but you have to put it in the sending unit for the right tank as they are not the same shape.
I would bet your rear pump is bad. You don't need to bypass anything to test it out. Test for power and ground at the plug for the rear tank to verify this before swapping stuff out. These trucks usually get leaks where the fuel tank straps go. Rusts them out. New tanks usually run a hundred bucks. Likely to need new straps and lock ring if you are just going after the pump, and even then you may have enough corrosion where the sending unit goes in that it won't get a good enough seal and will leak fuel badly. With trucks of this vintage I usually just toss the tank and straps rather than deal with the future leaks. You know your front pump runs, you could install that into a new tank and be good to go. Just last month I had a 93 F-150 with the same issues you are looking at and the "good" rear tank was only holding by a thread. When I unbolted the straps to lower it down the tank sprung a leak. It was pinholed bad where the straps ride and disturbing the rust made it give way. It was about $130 between a new tank and straps.
I hope the 8mpg is in-town. I've owned a couple of the trucks you have and was right around 13 highway.