So I bought a new gas cap a while back and it was supposed to be a vented cap. However I've been noticing that when the truck is sitting outside it builds up pressure in the tank. And it also builds while driving as I have an electric fuel pump. Shouldn't a vented gas cap prevent this?

 

Yes, a vented cap makes the inside of the tank the same as outside atmospheric pressure. No pressure should build up and no vacuum should be created by the fuel pump drawing fuel out because it *should* be immediately replaced with outside air drawn in through the cap.


A one-way vent is not sufficient. The cap has to be vented both in and out. Otherwise it won't work correctly.


The downside is, of course, as gas expands if it reaches the top of the tank and pushes up the filler neck it will spill down the outside of the truck. John can share the tricks for how to avoid that.

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Took the cap off and drilled a 3/32nds hole at the top of the cap under the rubber seal. Left it out in the heat for about 20 minutes and when I removed it there was still a bit of air escaping but I dont think it was nearly as much as before. I'll leave it out for another 20 minutes or so and see if there is a noticible difference.

 

This is a common problem, caps that vent out to the atmosphere were outlawed in the early 70's. So called vented caps they sell today only allow air in. I fought this problem on my non vented tank in my Ranchero, finally added a 1/8 hole, same trick many early Chevy guys do I found out. Hope the EPA doesn't read this LOL.

 

B6TZ-9030-A .. Fuel Cap-Vented Anti-Surge-Chrome (Motorcraft FC-23) / Reproduced - Also available NOS and from auto parts stores.

Applications:

Use with in-cab fuel tank: 1956 F100/950 / 1957/67 F100/1100 / 1963/69 N/NT500/1100 / 1967/69 F800/1000 / 1968/72 F100/750 except 1970/72 F100's with Evaporative Emission.

Use with 25 gallon factory installed auxiliary tank: 1967/72 F100/350 except 1970/72 F100's with Evaporative Emission.

1966/70 Bronco except 1970 with Evaporative Emission.

Evaporative Emission (non vented fuel cap, unleaded fuel) introduced in 1970, was mandatory initially in sold new in CA 1970/72 Passenger Cars; F100's; Bronco's and Econolines. Other states may nave mandated it as well.

By 1979, all states mandated Evaporative Emission.

 

Checked mine again yesterday and not sure if its venting enough or not. I might have to enlarge the hole or drill another one thats not covered by the rubber seal. Any thoughts or ideas?

 

Anybody? I drilled two more holes and its still not venting like I think it should. Has anyone drilled a small vent hole in the top of the filler neck?

 

i'm no expert, but this sounds like a REALLY bad idea.

 

I had the same issue with a "vented" cap that didn't vent both ways. I found this one for just a few buck at the local auto parts store:

http://www.pepboys.com/product/details/203146/00840

Solved my issue (stalling)--doesn't build pressure or vacuum.

~Steve

 

I'm with you. Building pressure in the tank shouldn't be an issue for fuel starvation. If anything, the other way around. As long as it lets air in to replace the fuel being pumped to the motor, you should be good.
An open vent hole in the filler neck sounds like an invitation to a BBQ in case of a wreck.

 

Before I replaced my cap with one that was fully vented, I was getting pressure build up in the tank except when I drove long distances. In that case, the pump would eventually starve and the engine would die.

~Steve

 

I checked with Pepboys and this is an anti-surge gas cap. Not sure if it vents both ways. So sseebart, did you have issues with gas pressure building up inside the tank while sitting in the sun and this cap solved that problem?

 

Yep, had that problem and this solved it. What do they mean by "anti-surge"?

 

Still don't see how pressure build up in the tank will cause fuel starvation Pressure in the tank should make it easier for the pump to suck the fuel from the tank. As long as all your hoses(including filler neck)are good, shouldn't have any fumes either.
Anyway, I've had a one way vented cap on my truck since '05, with both the original 6 cyl and the current 302 and never had a fuel supply problem(that wasn't caused by a bad float). Not about to change it at this point in time.

 

Running on the freeway, gas consumption is so quick that a vacuum built up in my tank (this is a 62 with a 292 in it). I found that if I ran it from full to about half a tank without stopping, the truck would reliably stall. (I did this about 4 times in the course of a couple of weeks). Pulling the cap after a stall, air would rush in. Let the truck sit for a while in the sun and open the cap, air would rush out. The cap I was using at the time (on the left in this photo) apparently didn't vent much either way. The one on the right I use now with no issues.

~Steve