, Hello guys,

Been wanting to post for a few months about this fuel leak I have but did not know enough to even make a post. I have a 1986 2.9l with twin fuel tanks. I NEVER fill the back tank its filled 1/3 of the way with rusty ancient fuel... but somehow I fill the front tank up and its somehow transferring into the rear tank and leaking out of the top of the filler tube just behind the gas keep of the rear tank!!?? (yet never seems to fill this tank either, how is this even possible?)

So I am getting about 8mpg with this leak... and just today I saw where it was leaking. How in the blazes is this happening!? There was also a leak on the top of both fuel tanks through old rubber gromets....which replaced. My brother said the small "fuel tank vent valve" has a ball or something in it if it fails fuel can bypass it and go to the rear tank, is this true? Here is a picture of the part that inserts into my new rubber fuel tank gromets..

https://shop.advanceautoparts.com/fi...nt-valve.c7145

Sadly, as proud of I am not being a mechanic and replacing both inline and tank fuel pumps, It never leaked until I "fixed it" now I am using a stupid amount of my salary right now on fuel. I am glad to see where it leaked though..

I don't know how dual tank systems work but there is a small line connecting both tanks.. Any ideas

Thanks for your help!

KnightOwl

 

No, not the tank vent

On a Dual tank system, 1997 and earlier, there will be a 6 port Switch in the frame rail
2 fuel lines from each tank to switch, for 4 ports and then 2 ports out to engine bay

Looks like this: https://www.f150forum.com/attachment...ectorvalve.jpg

Or this: https://i.ebayimg.com/images/g/egoAA...UV/s-l1600.jpg

Your 1986 uses a Return fuel system with a Fuel Pressure Regulator(FPR) on the engine
Each tank has a OUT from fuel pump line, and a Return from FPR in engine bay line

When the dual tank switch is activated it should change both OUT and RETURN lines to selected tank
Your current switch is not change the RETURN line or its valve is leaking, so some of the RETURN fuel is going to rear tank

You can remove the switch and just connect the 2 lines you need together, leaving rear tanks line unhooked
The OUT lines will be larger than Return lines

Front tank==============|switch|===========(filter and pump)==================Engine
Rear tank===============|switch|

Remove switch
Front tank==============================(filter and pump)==================Engine

 

Thanks for the information,

I just got back home from the grocery store and will have to fix it here at home. The rear tank (that is never used and had very little in it) was spewing out gasoline passed the gas cap like old faithful geyser, like it was overfilled!, crazy!. So I take the multiport widget in the frame rail you sent my photos of is broken or malfunctioning and needs to be replaced? I'm pretty sure i know where its at as I have taken the bed off about 7 times in the last month and a half, it looks like the second link you sent... Deleting the back tank sounds wonderful about now...Forgive me for my lack of mechanical understanding, but the "switch" I can remove.. is this the in cab selector switch that says front/back? Or is the part in your photo... the selector valve the "switch" I can take out not even needing? Thanks again

KnightOwl

 

The switch in the frame rail that changes which tank the fuel is coming from and returning to