Auxillary tank overflow

[ald7973]

Hi all - Newbie here...had my 75 Bronco for about 3 weeks and learning a lot! I live in KC and it's been pretty hot for a few weeks now... important in a moment. Truck has a main and auxiliary tank and I put ~5 gallons in each from a gas can when I took possession (was shipped empty.) Was out driving yesterday when I ran out of gas in the main tank. Tried to switch to auxiliary but it didn't work...not sure why not yet. So put another 5 gallons in the main, and then drove to a gas station to fill to ~11 gallons total in the main tank. Today I drove to a friend's house to watch a game and the truck was parked with the driver side in full sun in 90 degree heat....came outside a couple hours later to the auxiliary tank freely flowing out onto the street from the fill tube/cap....had to be at least a couple gallons. Theoretically there was just 5 gallons in the auxiliary.... obviously it expands in heat, but does this mean I have a bad cap or it isn't venting correctly for it to be streaming out like the cap wasn't even on? I'm not sure what is "supposed" to be happening when things are operating properly....other than not spewing gas of course! I'm read a few other posts about filling the tank slowly....but the aux tank was filled weeks ago so thinking that's not the issue. Thoughts appreciated!

 

[lars]

A few more details needed. Is your Bronco carbureted or fuel injected? If the former, it's a pretty straightforward system assuming it still uses the stock brass mechanical switching valve located below the drivers seat.

That said, mine has stubbornly spit out the caps regardless of caps, filler nozzles, tanks etc (I've changed everything) for the entire 28 years I've owned it, if either tank is anywhere near full. Especially if the drivers side is exposed to the sun and double especially if it's on a side slope. Gas expands a lot when it heats up.

 

[ald7973]

EFI. Good to know about all the other factors...will definitely keep in mind the rest of this summer!

 

[lars]

EFI. Good to know about all the other factors...will definitely keep in mind the rest of this summer!

Aha. You will need to investigate how the tanks are switched. There are lots of strategies out there.

I used OEM style Ford electric motorized switching valves for 20+ years. Note "valves," not "valve." Because they would fail. When it happened, the supply would pull out of one tank but return to the other. Depending on how much fuel was in the tanks, the recipient tank would overflow. So nice driving down the road with fuel spraying behind me. Or simply dribbling out the cap. I finally found a manual valve. Plumbing nightmare but utterly reliable.

 

[DirtDonk]

Even my '93 F150 did that. A common failure was for the two valves (one in each tank) to fail to return to the correct tank.
They would pull from whichever tank you had selected, but in either position the excess fuel would flow into the side/front tank. Big mess the first time I found that!
The strategy after that (in lieu of actually fixing the problem that is!) was to always use the front tank first, then when switching to the rear, it would use the rear, but refill the front slowly.
Gave me a little extra switching hassle, but it worked once you knew it was an issue.

Might have to crawl under yours and find out a few things.
Such as, do you have stock tank(s)? Do you have in-tank pumps, or an external? Do you have a stock manual valve under the driver's seat, or an electric one mounted down low somewhere with return lines?
So as said, a few more things to follow up on.
Oh, but even if it's all sorted and working, you can still get gas expanding out of the neck. Even with a good cap, the pressure will sometimes just overcome it's seal.

In '75, there should be a full charcoal canister setup. Meaning that the tank, or tanks, are vented. Or should be.
And while they built more air space into the later tank like yours, it still might not be enough to contain all the expansion. Your front tank is likely just about 7ga max. While the rear is likely about 11ga max. Just about the amount you put in!

paul