Bloated gas tank

[dbenichou289]

Hey everyone,
Something that I have noticed recently on my '68 is that when the tank gets to below 1/4 tank of gas, the tank itself has a tendency of bloating (viewed from within the trunk). I can tell that it is a build up of pressure in the tank as I can hear it 'hiss' out the filler cap and when I open it slowly, it all goes back to normal. Has anyone experience this in the past or could it be this new combination of fuel that includes the mandated "no more than 10% ethanol"?
The tank is about 11-12 years old and was sealed internally against rust, etc...at the time it was installed.

Thanks for your thoughts in advance.

 

[AzPete]

Sounds like the venting of the pressure at the cap is not working. All the old tanks had vented caps. It does seem odd at the 1/4 mark though.

 

[Starfury]

Actually, not all old tanks had vented caps. Some, like mine, had a vented cap mating surface on the feed neck.

Still, a properly vented cap should fix the issue.

 

[Mustang_Charly]

Like this:

 

[B67FSTB]

Sounds like the venting of the pressure at the cap is not working. All the old tanks had vented caps. It does seem odd at the 1/4 mark though.

+1

 

[hivewax]

I get the hiss sound... comes from a tight gas cap seal. Don't light a match!

 

[AzPete]

I get the hiss sound... comes from a tight gas cap seal. Don't light a match!

That hiss is ok for a sealed tank. If the system needs venting, that hiss can be leading to a collapsed tank or fuel starvation problem.

 

[dbenichou289]

I have the same cap as shown by mustang_charly as well as the indented areas as illustrated. I don't seem to have any fuel delivery issues - I step on it, it goes fast...

AzPete, are there any other indicators to indicate a collapsing tank?

 

[AzPete]

You may not drive it enough at one time to allow for the problem to develop. As long as the system is designed to be vented and you get the hiss when removing the cap, it is not venting fast enough. The hiss you hear could be from air being pulled into the tank also.

I have seen tanks that would actually suck in so much that the fuel gauge would increase some just before the car stopped due to starvation.

 

[dbenichou289]

Well, it got my attention not too long ago, when out of coincidence I had to open the trunk before filling up and the top of the tank seemed to have expanded. My gut feeling was something wasn't right, but then I thought maybe I just never noticed the curve of the tank. Well I was right because as I opened the cap, with hissing, I noticed the top of the tank settle back down.

So now I am trying to determine how to fix this. replace the tank? or filler neck? or gas cap? or could it be the gas/ethanol mix that is now used. I researched into ethanol and found that it does have a tendency of expanding. I was looking to see if anyone else experienced this or if I should look closer into my hardware.

 

[AzPete]

I have never seen the new gas mix cause the problem you have.

If a tank is not vented, it will expand as the fuel warms up thru the day. If you have a completely full tank parked on a warm day, it can overflow from expansion. Again, proper tank venting is what you need.

If the cork/rubber seal on the cap actually fits into that small notch designed for venting, it may be sealing the tank by mistake. If you can try a vented cap as a temporary item, that would show if venting is the problem. Once that is found, you can solve the specific problem to your system.