Venting gas cans

What's everyone's take on this?

Obviously the vent needs to be open when you are running the motor, and it would be good to keep it closed for trailering.

However, depending on temp, it swells/contracts pretty drastically. For example, I went out on Sunday which was a hot, sunny day. Closed the vent after loading the boat. Went into the garage late that night to check the battery charger and the tank looked like a crushed beer can (ok I'm exaggerating there a bit).

It seems like the extreme swell/contract cycles will reduce the life of the plastic. The tank is in the sun now when using/trailering the boat, and after my planned conversion, it still will be in the sun. The boat is stored in the garage though. This is a 3 gallon Honda OEM plastic tank if it matters.

Thanks.

Mine is always out in the sun. I was always curious if once it gets to a certain pressure does it stop or could it actually pop somewhere.. following this

I always kept mine open just a crack or two...worked for the outboard and didn't let anything spill while trailering. Maybe that's bad to do but worked for me.

What does everyone else do?

The vent on my gas tank stays open 24/7.

take a quart jar, lid off, and fill it almost full with fresh fuel. Octane doesn't matter. Mark the level of the fuel, then let it sit outside for a few hours, in the sun works well.

You will probably lose about 20%-40% fuel volume from the jar. In the process, the jar gets cool-and if the humidity is high, it will allow moisture to condense in the fuel, which obviously settles to the bottom.

Same thing happens in a vented fuel can, though it doesn't evaporate quite that quickly. But it still evaporates. That's today's fuel quality-and nothing that you and I can do about it. It's designed for EFI cars and pickups-and not for small engines-or anything else that sits for more than a few days.

I have had good results from just dumping the fuel out of the can and into something that's going to be used, car, truck, lawn mower, etc. Whatever I need gas for at the time. Usually the car. So far, haven't had to clean a carb of my own in a couple years.

Something else that I think doesn't get much consideration is ventless cans that build pressure while the fuel line is hooked to the motor. I've seen times where the pressure inside the gas can will overpower the float valve and flood the motor.

And the old gas cans can let fuel leech through the plastic. The new EPA compliant cans and lines aren't supposed to-but they are quite a bit more expensive. Most of you who still have metal cans don't have to worry about it because they are vented both ways (pressure and vacuum). I'm sure the EPA would like to see those go away.....

I opened the vent today just barely. Should be enough to let the pressure equalize. I didn't consider the water accumulation though.