should there be a slight vacuum when you open the gas cap

I recently purchased a 91 metro. I replaced the metal fuel fill tube that runs from the fuel door down under the left rear tire to the flexible line that eventually runs into the gas tank. MIne was rusted from the salt that flings up from the rear tire.

I pulled a perfect one from a 91 metro in a junk yard.

I know notice that when I remove the gas gap, there is a slight vacuum
build up that releases.

The cap says to open carefully. Is this vacuum a safety feature to keep gas fumes in the tank?

Or do I have a problem somewere with not enough venting of outside air when the car is running?

Thanks

My '96 metro does this virtually every time I fuel up an almost empty tank. I believe it's not a problem.

RossT:
The vacuum you talk about is perfectly normal. The effect is the same when you put even water into a bottle and shake it. Maybe you did not notice it before because the old tube that you replaced had tiny holes which let escape that pressured gas.

Hope this helps you clarify out your doubt.

Regards,

MC.

Thanks for the information!!

Yea, I have a 97' and it's always done the same.

It's actually the vacuum from the engine. Your gas tank has a vacuum line that goes from the top of the tank, to the charcoal filter under in the engine bay, then to the intake manifold. It prevents pressure from building up in the gas tank.