[skunkfunk]

Hello everybody, I had an old user name years ago that I don't really remember so I'm posting under a new one.

Anyway, I've had this gremlin with my 1969 Firebird. I had some vapor lock issues that I fixed by installing an electric fuel pump (and removing the mechanical pump) just outside the tank. The return line would leak fuel after I turned the car off, so I capped it. Now, when I park it, I can hear air leaking out of the fuel tank cap. If I remove the cap I get a nice blast of nasty gas air. I've tried a few different caps, vented, non vented, and that didn't fix it.

Where did I go wrong here? I'm sure there's some sort of simple fix. Do I need to get one of those later model fume kits like the next gen firebird had and hook it up to the return line under the hood?

[Zimtok]

It sounds like your tank is building up pressure...

You may want to check your vent tube to see if it is blocked. Not sure on your FB where it is but mine is at the top end of the filler tube.

I had a similar problem with my 65 GTO and found out that the previous owner had plugged the vent tube because it was leaking fuel every time he accelerated.


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[skunkfunk]

I have in the past had problems with fuel leaking out under the tank. I had figured that it was the sending unit, but since it holds pressure it obviously can't be that. Could very well be the vent tube. Assuming it is the vent tube, and assuming that if I unblock it I will leak fuel when the tank is fullish, what would you do?

[Zimtok]

You're "assuming" too much.
You need to get the tank vented to prevent the pressure building up.

If there is some kind of leak after you get the tank vented properly, then yhou can look for that leak and fix it.

My guess is that the vent tube was leaking fuel because there is something wrong with it and it may have been plugged to stop the leak.

My vent tube was broke off and the Previous Owner had put a bolt in it to plug the hole where it had broken from the filler neck. This caused the tank to build pressure and eventually caused a leak at the joint where the filler neck attached to the tank. I fixed the leak and the vent tube. All is working great now.


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[NeighborsComplaint]

I am wondering how to vent my tank also (1971GTO). When I went to replace the tank, I found the previous owner had removed the charcoal canister from the ECS but left all the vent tubes connected into the car interior. Needless to say, that explained the fuel smell I was blaming on my Holley carb.

I get some pressure buildup in the tank since I capped off all the vents. Should I just run a vent tube up the side of the filler neck and vent to atmosphere or will a vented gas cap take care of it?

[HWYSTR455]

Yeah, this issue is a little more complicated than one would initially think, since we are now dealing with a more volatile fuel with ethanol. New pump gas heats, boils, and expands more rapidly than previous pump gas.

NC - 71 GTO, there's a separator that is behind the rear seat in the trunk, but, it's not exposed into the passenger compartment or the trunk. vent tubes run from the tank up to it, it separates the liquid from the vapor, and returns the liquid to the tank. Vapor runs up to the charcoal can, and then to the carb. The element on the charcoal can helps prevent any smell. If any of the system is not as it was when new, start there.

On a 69 FB, not sure they used a separator system at that time. That vented vapor/psi into the atmosphere via the cap, and it's separator was the line running back to the fill tube. On that system, you HAVE to make sure you get the right cap, or psi will build. Add a return/bypass fuel system to that, or use new pump gas, and it needs to be improved. Not sure how to do that, improve the system, I'm not that familiar with it, but it would probably take fabrication.

There are aftermarket fuel vent tanks, some do a form of separation, but those too generally vent into the atmosphere, so one would most likely get a fuel smell, especially when sitting in a closed garage.

I have similar problems, one is on the 442, since the Tanks, Inc EFI tank isn't equipped with all the same factory vents, and it's near impossible to incorp the OE separator. That stupid vent checkball/valve they ship with the tank should go right in the trash, spits fuel if you even look at it wrong.

Anyway, I'm going to come up with something, this is getting to reach epidemic proportions. Once I figure it out, I will post.

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[tom s]

My 69 had the problem.When I used a vented cap I would loose gas(aftermarket tank).When I used the 68 non vented that allowed air to be pulled in but not out it would build stupid pressure.I took the 68 non vented cap,and drilled a very small hole just above center,I marked a spot when the cap was on,just have to make sure its indexed right when put on.Tom

[Tom Vaught]

Originally Posted by HWYSTR455

Yeah, this issue is a little more complicated than one would initially think, since we are now dealing with a more volatile fuel with ethanol. New pump gas heats, boils, and expands more rapidly than previous pump gas.

NC - 71 GTO, there's a separator that is behind the rear seat in the trunk, but, it's not exposed into the passenger compartment or the trunk. vent tubes run from the tank up to it, it separates the liquid from the vapor, and returns the liquid to the tank. Vapor runs up to the charcoal can, and then to the carb. The element on the charcoal can helps prevent any smell. If any of the system is not as it was when new, start there.

On a 69 FB, not sure they used a separator system at that time. That vented vapor/psi into the atmosphere via the cap, and it's separator was the line running back to the fill tube. On that system, you HAVE to make sure you get the right cap, or psi will build. Add a return/bypass fuel system to that, or use new pump gas, and it needs to be improved. Not sure how to do that, improve the system, I'm not that familiar with it, but it would probably take fabrication.

There are aftermarket fuel vent tanks, some do a form of separation, but those too generally vent into the atmosphere, so one would most likely get a fuel smell, especially when sitting in a closed garage.

I have similar problems, one is on the 442, since the Tanks, Inc EFI tank isn't equipped with all the same factory vents, and it's near impossible to incorp the OE separator. That stupid vent checkball/valve they ship with the tank should go right in the trash, spits fuel if you even look at it wrong.

Anyway, I'm going to come up with something, this is getting to reach epidemic proportions. Once I figure it out, I will post.

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Subscribing on the Fuel Tank Vent info.

TV

[HWYSTR455]

Ok, so initial info, here's some OE diagrams of some fuel tank venting for 69, and 71-72. If you look at the tank, there's vent tubes on each corner in the front, and one in the middle. Reason is, so when the car is sitting on a hill, the vent isn't covered, and the tank can still vent without creating a syphon.

So in that separator, that goes between the rear seats, the tubes that the tank feed into are short, and the one that goes forward to the charcoal can is long, so the liquid that may find it's way up to the separator can flow back to the tank, and the vapor can go to the charcoal can, and then drawn into the carb.

The charcoal can is just that, it has charcoal in it, with a cotton/fabric bottom element, to allow any liquid that makes it forward to be expelled at a slow rate, and not smell.

The 69 has 2 vents, but both go to a Y piece that is open to the outside, and it's supposed to let fuel drain back through one of the 2 lines, but, since it's so small, if pressure builds fast enough, or the car sits on a hill, it can force fuel out. If psi in the tank builds fast enough, it will overrun the lines simply due to their' size.

One pic attached is an attempt at creating a separator using a radiator expansion tank. Not a bad idea, but, it's still limited to one return line outlet, and it's 3/8 or maybe even 5/16, not big enough.

Going to chew on this a bit, but one of the big problems is that early tanks, and the tanks inc tanks, only have one return. Maybe cutting the fill neck and using a EFI return adapter may be an option, again, need to think about this a while...

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[tom s]

My 69 firebird tank has no vents,my 69 GTO had one on the top pass side front of the tank.Tom

[HWYSTR455]

My guess is someone removed it, or You're not looking in the right spot. On the firebirds, the vent is in the back, and it routes through the trunk. Someone on this site actually extended it with a rubber tube/line so it wouldn't slosh fuel on acceleration:

http://firstgenfirebird.org/forums/u...&Number=210990

The line is available repop, the attached pic is off egay.

It still doesn't help, because I'm thinking there's got to be more than one vent, on diff sides/locations on the tank.

But I'm still chewing on this...

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[tom s]

My aftermarket firebird tank and sender has no vent.They make senders with both one and two lines.Tom

[Tom Vaught]

I plan on modifying the 64 GTO Tanks,inc Gas Tank to allow two efi pumps (in the tank baffled sump area) and when I create the top plate to do that, I plan on adding extra fuel tank vent lines inside the tank and have extra vent nipples located in the custom fuel pump top plate.

Tom V.

[HWYSTR455]

I think I'm going to buy another Tanks Inc EFi tank for the Olds, and have vents installed in the OE locations. Bet I can sell the one I have in there now, it's not very old.

May look into charcoal filters and breathers, might be able to make something work with one of those. Or just put the OE one back in, have to see what's available first. The OE separator works, but still interested in cars that didn't have them from the factory, and retro fit situations.

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[tom s]

I put the tank new in the car along with the sender.There is NO vent in either!There are senders avil WITH the vent.I did not buy that kind.Like I said before.Tom

[HWYSTR455]

Hm. Interesting. Well, that's a problem I would think, no vent. No way can a cap keep up with the requirement, they vent too slow. After parking it, and a little time goes by, do you ever hear a 'thump' or a 'thud'? Because it can suck a tank together, like collapse it, especially if you use fuel fast enough.

Ever have a delivery issue? Once a certain amount of vacuum builds in the tank, the pump (mech) won't be able to suck any fuel. If it's electric, the pump can overheat.

Tank vents need to be about the same size as the feed line, fuel system 101. It actually could be a little bigger, since under pressure, the feed line can flow more than something just open to the atmosphere, and vacuum could build.

I found out that some GM brake boosters use a charcoal filter to prevent fuel vapor from destroying the booster diaphragm. (pic attached) It's a Dorman part number 80195. That would prevent any fuel odor from escaping.

I've also heard some use a fish tank filter, same thing, charcoal, but haven't found any that are the proper size, and can handle fuel.

I was looking at marine vents and check valves, there again, some of the sizes are too big, or too small, but would image there would be some in between that would work, just will take a little digging.

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[tom s]

Had no pressure build up with the vented cap ever,just leaked gas when ever you left a stop light.Crap aftermarket tank!I think the filler neck was too flat.Tom

[78w72]

Originally Posted by tom s

My aftermarket firebird tank and sender has no vent.They make senders with both one and two lines.Tom

1970 firebird tanks have no vents either. i have a 72 firebird that i removed the EEC vapor tank & lines & didnt plan on using the charcoal canister in the engine bay... i bought a new repro 1970 tank which has no vents at all.

i used a robbmc 1/2" pickup & it has a return line in it... robb said to just use that as a vent if i dont have a return. i ran a hose form that up above the tank with a plug in it & drilled a very small home in the plug. i use the correct 1970 inward vented cap. no more pressure build up in the tank, but it does stink up the garage a bit after a drive, seems to mostly go away after that first day. before drilling the plug it would build up pressure pretty bad.

[HWYSTR455]

Originally Posted by tom s

Had no pressure build up with the vented cap ever,just leaked gas when ever you left a stop light.Crap aftermarket tank!I think the filler neck was too flat.Tom

Hm. If it leaks gas, it leaks air, and bet that's why there was never a build up.

The neck doesn't have a tube/fitting in it?

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[HWYSTR455]

Originally Posted by 78w72

1970 firebird tanks have no vents either. i have a 72 firebird that i removed the EEC vapor tank & lines & didnt plan on using the charcoal canister in the engine bay... i bought a new repro 1970 tank which has no vents at all.

i used a robbmc 1/2" pickup & it has a return line in it... robb said to just use that as a vent if i dont have a return. i ran a hose form that up above the tank with a plug in it & drilled a very small home in the plug. i use the correct 1970 inward vented cap. no more pressure build up in the tank, but it does stink up the garage a bit after a drive, seems to mostly go away after that first day. before drilling the plug it would build up pressure pretty bad.

No vents? I don't get it. Think about it, you remove gas from a tank through a 3/8 or 1/2 line, it has to let air in at the same rate, or vacuum is going to build. Fuel tanks in the ground keeps fuel cool, you pump it into the car and not only does it get aerated, but as it heats, it expands. It's got to go somewhere. And the caps are certainly not enough flow for a performance app.

I've seen people have delivery issues with tanks that are not vented properly. I've also seen tanks spit fuel when the cap is removed at the pump. This is not something that should be ignored, and needs attention just like any other area of a fuel delivery system.

Just because it 'seems' to be working doesn't mean it's working optimally. Just like how some don't know they have a minor fuel delivery issue until they upgrade, and all of a sudden they pick up at the track or whatever.

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