I thought this one would be simple to diagnose, but here I am.

I have a midship tank and a rear tank. (I think the rear one is the main one right?)

Anyway both of them are super hard to fill, I have to trickle fuel in. The rear tanks, the fill tube doesn't seem collapsed and if I stick a 3/8 fuel hose into the vent tube I can easily blow through it and see the fumes coming out the filler. There's no kink in the fill tube.

When I do that same thing on the midship tank (gauge isn't working yet, but sounds empty when I knock on it) a lung full of air through the breather gushed a lot of fuel right out the filler. This filler has a noticeable kink in the fill tube.

So I've got 2 tanks with the same symptom of being hard to fill, the midship one seems simple enough to fix the kink, but I wonder why I had fuel gush out when I blew into that one, but not when I blew into the rear one. Maybe there's just more fuel in it than I realized. I jumped the wires at the sender and the gauge pegged, that tells me the gauge and wiring are good, but the sender is bad. correct?

Do these tanks have a skin around them, so when I'm knocking on it, I'm not actually knocking on metal that has fuel on the other side and it'd seem empty when it's not?

Sorry for the weird questions.

 

The rear is the main tank.
It takes a lot of vent when filling the tank. Think about it, you are shooting about a 3/4 inch diameter stream of gas going in, you have to have about that much or more stream of air coming out of the tank.
Take another look to see if the fill hose is compressed too much.

 

when you switch from the main to the aux tank...or vice versa... the selector valve switches which tank the gas is drawn from and the switch in the dash also switches over to the corresponding fuel tank sending unit so that you can tell how much gas is in the tank, make sure your switch is operating correctly in the dash...the vent hose also cannot be kinked or obstructed...I used to have an old junk f250 4wd farm truck that I would have to have the d/s rear tire higher than the others to fill it up..never troubleshot that problem..just ran it 'til it died....of course that was 'some' years ago ..

 

The filler tube for the main is ovaled at best (it's not squashed, just barely ouy of round)
The filler tube for the Aux tank has a kink where it bends, that kink is about 20% restriction.

both air vents free of kinks and debris, what concerns me the most about the vents is how the main one blew out fumes like it's supposed to, but the other gave me a lap of fuel. Common sense is saying that maybe that's actually a full tank. So I'm driving on that to empty it so I know for sure. Gonna start hunting for a new sender in the meantime.

thanks for the input.

 

I have the same problem with a 74 F100 and a 65 Falcon, I have no problems with kinked hoses or blocked vent lines. I am pretty sure it has something to do with the bend of fuel filler neck and the bend of the gas pump noozle. I have one gas station in my area I can go and fill up with no problems, any others it a 10-15 minute ordeal to fill up and fuel running back out.