Thread: Gas tank pressure?

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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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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.

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.

That will do it...