Fuel Tank Vacuum?

[jemartin144]

Hello, so I just acquired a Festy and am in the process of going over it to make it more road worthy.

I noticed something the other day when I went to first fill it up. When I take the fuel cap off theres quite a big vacuum in the tank. I thought maybe its because its been sitting around for a few weeks and there was a change in pressure due to temperature or something, but I took the cap off again after driving it only a few miles just to see and it seemed like it pulled just as big of a vacuum. I've never noticed this before on a car, but Ive never had a car this old either, so i dont know if its something thats normal or if I should be concerned?

I've always thought there is a vent with some sort of check valve on fuel tanks.

[Tommy]

I fill up twice a week and it does that every time, I always thought it was weird because Ive never had a car do that before.

[Safety Guy]

Mine will do that sometimes, but not all the time. I have no idea what the circumstances are which cause it, but I do know the system is supposed to keep gas fumes from escaping into the atmosphere for pollution control.

Karl

[FB71]

Its not a vacuum, its pressure. Usually less than 1psi. There are vent valves to both atmosphere and the evap canister, but both will hold just about 1psi before they vent.

[Pu241]

FB71, beat me too it.

It's only about 1 psi but it's over the unfilled volume of your tank which can result in a fairly protracted movement of gas (not gasoline) through the fill port.

Also since the pump is cooled by the liquid fuel in the tank, on a hot day and only a small amount of fuel in your tank, you may well be building pressure fairly quickly.

[skimsucka]

In hot fl my tank does this everytime but i knew what it was

[jemartin144]

Thank you for the responses. Looks like the answer is to not worry about it. Thats one more thing checked off my list.

Thank you.

[zoom zoom]

I always liked that part of the car, i almost want to make it hold up to like 5 or 10 lbs of pressure, probably get more miles per tank runnin on those fumes!

[Oren09]

My truck does this really bad when it's warm outside. So bad that liquid gas starts leaking out of the seal where the fuel pump module goes into the tank. I just leave the gas cap off and it helps keep the pressure down. My truck is barely OBDII (It's a 97) and it doesn't monitor the gas cap.

The Festiva does it as well but no near as bad and doesn't leak as far as I can tell.