Hey everyone I have replaced both fuel tanks on my 1986 f 150 just recently i noticed fuel coming out of the rear tank and mt front tank is empty within 5 miles or so . any clue of whats going on?

 

My guess is the tanks weren’t installed properly......ie: maybe a hose/line connection isn’t tight, a seal is missing or kinked, etc.

only thing to really do is re-check everything.

 

What engine? What fuel system? If this is an EFI system, it has a tank switching valve and fuel return lines to use with electric, in-tank pumps.

 

And when you say the front tank is empty in five miles, is that the gauge reading only? Or is it sucking air after only five miles and you know you put a full tank's worth of gas in it?
If the gauge, then maybe the sender is not working right. You'll have to wait until you run it dry to know for sure. Did you install new senders?

If it's sucking air in five miles then your pickup tube is wrong (sitting too high) or clogged somehow. Or something else, like the fuel selector valve is failing from heat or something.

Definitely an odd one.
But gas leaking out of a cap is just a leaky cap. Is the cap new? Is the filler neck new? They're correct for each other? If so, then one has a defect. OR, you're having pressure buildup because the EVAP vents are not functioning. You didn't happen to remove any of that stuff at the same time by any chance, did you?
Were you the installer? Or did someone else do it for you? If someone else, you're going to have to crawl around under there to see if any hoses are missing or disconnected.

Were they factory replacement tanks, or aftermarket designs? Larger, or maybe pre-smog era types? Lots of information still to know.
What was the reason for replacement in the first place?

Good luck.

Paul

 

Selector valve screwed up? Return line going to rear tank instead of back to the front tank?

 

That is what I am thinking the returns are going back to the wrong tanks or the valve is messing up.
But we need to know what motor & fuel system you have.
Dave ----