Does the vent in the after market tank (JAZ?) cap vent air in or out? Thanks WB:(

Hey Tinker,

I just saw your thread from back on the 11th and since no one bothered to answer you, I thought I would. It makes me crazy when some of the simplest of questions go ignored.

Original equipment or aftermarket replacement, it doesn't matter - they both work the same way. You didn't say what car you have so I'll make it as simple as possible: Cars with EEC (Evaporative Emissions Control) use UNVENTED gas caps. That's the cars with the charcoal canister in the engine compartment. Cars without EEC utilize VENTED caps. The vent allows air INTO the tank so as gas is pumped out, a vacuum is not created in the tank.

vented caps allow air into tank and allow some fumes out of the tank.
if you pull a vacuum on the tank it can collapse inwards...........

Vince,

I have an issue with pressure building up in the tank of my '70 Elky (vented cap) and would love to find one of those vented caps that "allow some fumes out". Whose brand and where can I buy one? Every vented cap I've ever had only vents one way.

There is vented and then there is "vented"...

Any vented cap will allow pressure to pass either way. The rub is at what level the pressure is allowed to pass? If you think about it there is not point nor reason to pass pressure out of the tank if it is less than the fuel pump line pressure. Thus there can be some amount of pressure present in the tank.

You really do not want a fully open vent on your Elky cap unless you like spilling fuel on right hand turns. BTDT. When I replaced the tank in my '68 I went with the EEC version of the tank for it's better venting. Then I grafted in a late model pickup fuel filler with a sealed anti-surge cap. It is great because now I can stick the fuel pump filler in and it latches in. Before I had to stand there and hold the darn thing doing the "Elky Lean".

Hey Steve,

I've go two vented caps here that say you are mistaken. They'll only allow air INTO the tank, not out. If they worked both ways as you say, I wouldn't have a gigantic rush of air blow out when the cap is unscrewed. If such a two-way cap really exists, I and a lot of other guys who experience pressure build-up would sure like to know where to get one.

Read what I posted again. There is a substantial volume of air in your gas tank (especially close to empty) and if pressurized to only 5 PSI above atmospheric, you will note that gigantic rush. My point was that the caps do vent out but only above a preset pressure level.

Steve,

The question still remains. Tell me where to get a gas cap that vents both ways - even a little bit. You'll never find a single thread at Team Chevelle that has ever mentioned a vented cap working both ways and that's with a lot of threads talking about gas tank pressure buildup. I don't understand what you are basing your assumption on.

whoa........
didn't mean to cause a big misunderstanding, most vented caps appear to have a check valve mechanism to only allow air in, as allowing gas fumes out could result in a fire.
i have an aftermarket fuel tank with a vent line/hose attached and guess it should have one of those check valves on it as air/fumes can go in and out. i have also been conserned about dirt/dust being drawn in thru this vent line, maybe i should put a filter on the end?

I put a new gas tank on our '69 el Camino with a new GM vented gas cap. Some times there is considerable air pressure when I remove the cap. Everything seems to work fine though.

I tried an expirement with a vented anti-surge cap I had on my tank. I could *always* suck in. I could only *sometimes* blow out.

This was just using mouth air pressure which I consider inconclusive.

There is definitely a check valve and it would take an air compressor to validate if there is not a pressure relief valve as well. I know few people who can exceed normal fuel pump pressure there a relief valve would have it's threshold pressure set.

PS: yes putting a filter on an open vent line is probably a good idea. The last thing you want is some bug crawling up inside the line where you cannot see it and causing problems.

I have a 70 chevelle and have alot of pressure build up in my gas tank too. I've tried different vented caps since I have a non-vented tank which has 2 hoses coming from the sending unit, one of course is the gas line and the other one which is smaller I don't know where it goes? The car does not have a charcoal canister.

thanks,
Tim

Does your tank have a FUEL TANK VENT 1968-72 like the one in my 69 chevelle? the part im looking at is XTV-689 in the ground up restoration site and goes to two hoses on the right front corner of my gas tank, that device may be the problem your experiencing???

I posted this solution to my '70 tank pressure problems in the Chevelle Tech forum the other day.

http://www.chevelles.com/forums/showthread.php?t=186510