Gas tank pressure?

This should be a simple fix however, I must be pretty simple because I can't seem to find the solution. This has happened several times now including once that blistered the paint where the gas ran out.I d
Underneath my '36 chevy sedan is a poly gas tank...I don't fill it up completely...it is vented with a hose and screened vent cap that is not plugged or at least I can blow air in and out of the hose easily. When I do that I'm blowing air into air at the top of the tank, not into gas. I've changed gas caps and put a new push button type to release and pressure before opening. Here's the problem: gas pressure builds in the tank and spews out the vent as well as the gas cap even after the car has been parked in the garage for several days garage door closed or open, it doesn't seem to matter about the air temp or sunlight that actually never gets near the tank. Do the poly tanks expand and contract that much? I installed a drain tube where the push button on the gas cap used to be hoping that will prevent gas from dribbling down the side to the car....Any thoughts or suggestions would be most appreciated.

Do you by chance have an in tank electric pump with a return line regulator up by the motor? I've heard the these regulators sometimes get dirt in them and stick, returning too much pressure back to the tank, even after the motor is off. Something to check..

I think you will find that the solution will be to fill the tank less full than you are currently doing. It's not the tank that is expanding/contracting so much as it's the fuel itself. It sounds like your venting system is functioning correctly and the the tank itself would have minimal contraction/expansion. That pretty much leaves the fuel itself.

I'm going to take a guess and say the problem occurs after a recent fill up. Fuel coming out of an under ground tank is usually a lot cooler than it is once it's above ground in a cars fuel tank, so it's going to start expanding once it's in the car. It may take a couple of hours or even days (depending on the outside temperature), but it sounds like once it does expand you are basically just in an over full condition.

It may just be my imagination, but I think the current fuels that have ethanol add seem to expand more than the fuels we used to get without the ethanol.


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An in tank fuel pump will all result in the fuel being warmed up which causes the fuel to expand.

Your vent may have a roll over valve which is sensing the rising feul level ( expansion) and closing your vent.

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Originally Posted by firebird77clone

Your vent may have a roll over valve which is sensing the rising feul level ( expansion) and closing your vent.

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Installed correctly, the anti-rollover valve sits at or slightly above the top of tank, where it doesn't see liquid unless the system is in a rollover situation. A short brass nipple with a coupling elevates the valve. Too many people simply screw it into the tank, and then wonder why their system is building pressure. Like firebird says, the rollover check will float up, sealing the system if the fuel level is over the bottom of the valve.

If it is building pressure, then the venting is working. So it must be the cap and the roll over / vent.
I have a poly tank and have never experienced what you're describing plus I don't use a vented cap.
I have to agree with the others, verify the valve is free and not having gas splashed onto it causing it to close.
You may have to relocate the roll over valve to keep it clear of the fuel surge..

Thanks, Mike P...30_40 also had a interesting tip with the non-vented gas cap. I'm suspecting that your right about the cooler temperature of "fresh" gas allows more fuel to be added than should be...however, the second time excess pressure built up, the car had been sitting in the garage for two days before starting to spew out the relief line which is located just above the filler tube. I had driven the car for approximately 10 miles prior to parking it so I know there was "some" amount of air space inside the tank. I also tried a non-vented gas cap but the pressure came out the vent tube along with a small amount of gas so I drilled the cap and installed a small tube which routes thru a drain tube at the bottom of the gas cap area. since then, about 1 week, there has been no pressure. Have my fingers crossed on this on...Thanks to all the Forum members for the suggestions and advise.

Maybe a cause/solution...I have a Summit roll-over valve installed right over the filler tube but...it's installed upside down.
Since I vented the gas cap and relieved tank pressure that way, I think my roll-over valve is pretty much non-functional now however, I'll try and relocate it right side up. Thanks again for all the help.

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Originally Posted by 1936CHEVY

Maybe a cause/solution...I have a Summit roll-over valve installed right over the filler tube but...it's installed upside down.

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That will do it...

sometimes when your leaning over a fender or laying under a car its difficult to remember whats up and whats down----

I once put the high speed lean out valve upside down and we lost the race-----gave the world championship to Jerry Ruth

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Originally Posted by jerry clayton

sometimes when your leaning over a fender or laying under a car its difficult to remember whats up and whats down----

I once put the high speed lean out valve upside down and we lost the race-----gave the world championship to Jerry Ruth

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That had to have been a while back as I haven't heard that name in a stone's age:D:LOL::LOL:

73 or 74? I had been flying all night the day before and then to Amarillo for NHRA World Finals -John Stewart driver, Bill Schultz----Amarillo was 4000 altitude I think-we had run good there because I used a lower gear for more revs-car was nosing over towards top end so wanted to go more on jet size and earlier with high speed lean out valve-it was installed downwards in the Hilburn pump-I took it off (not normal) because I had to change jet and shims,etc------tired as all hell-put it back in upside down so it never opened at all and car nosed over about 800 feet instead of 1000-1200

Lessons learned---call in sick for flights before race days not just the ones on race days

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Originally Posted by jerry clayton

I once put the high speed lean out valve upside down and we lost the race-----gave the world championship to Jerry Ruth

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OUCH!!!! :whacked:

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There's more than one type of rollover valve. Some are made to be remote mounted and others are made for in-tank mounting. I have a tank mounted model, but for some reason it says they work best with a short filler neck. My filler neck was short at one time, but now it's at least 3 feet long and a lot higher. Maybe I'll switch to a remote model. The problem there is getting it high enough and vented outside the car. I'd need a hose on both sides of the valve to accomplish that.

Remote Mount Rollover Vent Valve

In Tank Rollover Vent Valve

Tech 101: The Hows and Whys of Installing a Tanks Inc. Rollover Vent Valve - OnAllCylinders

The check ball in a rollover valve is steel and won't float. When upside down, it will sink and do it's job.

If a tank builds up pressure, the rollover valve check ball may be stuck closed. A vented cap should not be need in addition to a rollover valve that provides venting.

Getting any of that snow Dave????????

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Originally Posted by jerry clayton

Getting any of that snow Dave????????

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Only a small amount and it all melted in one day. We're more concerned about the temp staying above freezing since many flowering trees and bushes are just now blooming. A good freeze would spoil the flowers.