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vented gas caps?

Posted: Sun Jul 19, 2015 10:29 am
by Stan
Ok you Bronco Brainiacs - I've got two vented SS gas caps. {feel free to insert jokes anywhere you want } One I can suck air thru/into AND blow air OUT of. The other I can only suck air IN thru the vent but can't blow air out . So - which one is correct . I've got the two "old ones" buried somewhere in the spare part pile and behind the pile of current Bronco projects pile or I'd check those out to see how those vents operate.
I'll spare you the details but it has to do with gas dripping out of the cap that I can - blow and suck thru - as in melting the clear,paint,flex additive and primer off of the fender flare. I should add the tank is at minimum 2 gallons below being full - so the pressurized gas fumes must be pushing the fuel up an out the vented cap.
I've got 3-4 ways to repair the flare paint but don't want to fix that if it'll happen again --- so I'm wondering is it a fuel cap issue - a tank design issue or a combo?

Posted: Wed Jan 20, 2016 8:07 pm
by DirtDonk
Seems to me that the one you can't blow through is for a mid-'70 and later model with the charcoal canister evap system. Those are supposed to let all fumes go through the canister and get sucked into the engine to be burned. Any cap that allows venting to atmosphere is a pre-smog cap that would allow expansion to go out the vent and into atmosphere.

When they changed systems, they also changed the depth of the tangs that engage the slots too. So an old cap would theoretically not be able to work on a newer Bronco.
And the '77 was completely different as well, and even earlier smog caps won't work at all on a '77.

Now all that's just theory, and the one you can blow and suck through might just be defective.
It's not the cap alone that keeps the expanding liquid from leaking out. Most won't stop mildly pressurized liquid from seeping out eventually. Even when they're new and working perfectly.
As many of us have found out over the years.


Posted: Thu Jan 28, 2016 10:09 pm
by TonyNokes
Mine has ruined the paint below the fill necks also... If you figure it out let us know.