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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