I have a 1994 f150 with a 351. The rear tank stopped working and it wasn't the switch so I replaced the pump and in the process found out I had a hole in the tank so I replaced the tank and pump but the tank I put on had a different vent valve and I didn't realize until I had already got rid of the old one so I just used the one with the one hose hookup. After installation the rear tank worked perfect for about 40 miles then just quit, it would idle but as soon as I gave it gas it would sputter and die. I noticed that the pump sounded really weak so I replaced it a second time and it ran good for a little while but now it is really sluggish and sputters when I try to accelerate but only on the rear tank. Only thing different then I use to be when it worked is the vent valve so I found one with the duel hose hookup at a scrap yard and replaced it but it still won't run rite on the rear tank. It acts like it's starving for fuel. Is it possible that having the wrong vent could have ruined the pump? Any information is helpful. I'm out of things to try.

 

You need to verify the fuel pressure on the fuel rail when using the rear tank. The failure rate of aftermarket fuel pumps is alarmingly awful. Multiple failures in a short time is common if not using a Motorcraft replacement. On the other hand since you have a side mount tank it could have a bad check or shuttle valve which bleeds off pressure to the fuel rail when using the rear tank. The latter is referred to the very common crossflow issue these trucks are prone to have. The most common symptom besides your stated driveability issue is the unused tank mysteriously fills with fuel.

 

Do you know the vent works?

As a fuel pump pulls fuel out of a tank the tank needs to be able to get air into it to replace the volume of fuel pulled out. If it can't "pull" air in then a vacuum is created and the pump isn't just pumping fuel to the engine it's fighting the vacuum in the tank to get that fuel out. Try loosening the fuel tank cap to break the seal and see if it will run. Or it could be the cap not allowing air in (not sure how that works on your truck.)

Had a brand new Honda 4 wheeler years ago that would fire right up. Ride it a few hundred yards and it would die. You could crank it all you wanted and it would splutter/not run. Pop the gas tank cap off, put it back on and it would fire right up and you could ride it another few hundred yards before it would die again. Took the cap back to the Honda place, explained what was happening and they handed me a brand new gas cap - no more issues.

Not saying that's your problem, but the tank has got to have a way to replace gas with air.

Good luck with it.