You made me go look at mine. It's the same as the one on the left. I also verified that it only vents one way. I've driven my truck on the freeway, pulling an 18 foot travel trailer as in my avatar and also once even with a car trailer hauling a F-250 Camper Special(yes, I know it was way overloaded). Never a problem. Who knows?

 

DONT DRILL ANY HOLES IN NECK WITH A POWER DRILL !!! EXPLOSION VERY POSSIBLE

 

Dont worry, not drilling any holes.

 

It has to vent both ways or one of two things would happen. Vacuum would exist from the pump removing fuel that can't be replaced with air or pressure would exist from gas expanding from the suns heat or parking in a warm garage. If you have a completely unvented cap then both can occur, although not simultaneously in most cases.


I assume you are using your mouth to test it. Try this. I did this just this morning with one original gas cap and two Ford B6TZ-9030-A (Motrocraft FC-23) replacement gas caps. Visually they all have similar construction on the inside to sseebart's right hand one in his picture.


Turn the cap right side up like it is installed on the truck. All of mine allow breathing in and out through the cap. Now turn the cap upside down and try the same thing. Only vents one way, right. That's the anti-surge check ball. Shake the cap and you can hear the check ball moving.


The idea there is if you go around a right-hand corner quickly and the gas surges up the neck that the check ball will already, due to centrifugal force, have rolled to block gas from spitting out. But, if pressure slowly builds up from heat from the sun it is able to escape.

Can also happen if it's really cold outside, you fill up the truck and then go park it in a warm garage.


Gas is usually stored below ground at the gas stations and so is typically cooler, like around 55 degrees or whatever the surrounding earth temperature is. You fill up your tank on a hot day and that gas is going to expand. It has to go somewhere. The tank is sealed except for two places. The gas cap and the line out to the fuel pump. The line out to the fuel pump doesn't count unless the engine is running (under normal operating conditions).

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I went out and tried that. Made my guys look at me weird. Cap is still vented in only. Will not release pressure right side up or upside down.
I believe the one way cap was probably EPA mandated at some point in the '70's or '80's to control gas fume emissions, like those big gas nozzles. I sorta remember something about that in the dim recesses of my mind from back in the '80's when I held a Calif Smog license.
I suppose it may also be that it releases pressure into the atmosphere at a pressure higher than I can obtain with my lungs. Either way, mine's never been an issue.

 

I'm curious what happens with yours if you fill the tank full and let it sit in the sun. If pressure doesn't built up in the tank then it's got to be venting. But, it's just a curiosity. Mine all work as originally designed since they are correct Ford caps. Certainly I won't lose any sleep over some one else's gas cap....Lol.



They did make unleaded gas nozzles smaller because they made the openings in the gas tank fillers smaller. That was all so you couldn't easily and accidentally put Ethyl or Leaded into a car that expected unleaded or it could damage the catalytic converter. Once most stations eliminated leaded gas all the nozzles went to the smaller size.

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It's amazing how such small and seemingly insignificant parts can cause such huge problems

 

Went to pepboys and bought the cap listed above and it works like a champ. Left the truck out in the sun all day yesterday and no pressure build up. Drove it about 50 miles yesterday evening and opened the cap when I got home and no negative pressure. I think I've solved my problem!!

 

TA, I was talking about the gas nozzles with the bellows on them. The bellows is supposed to seal to the filler neck and collect fumes caused by refueling.
May only be a SoCal thing, as they don't have them in Arkansas.