SC400 Gas Tank Pressure, Is It Normal?

[moonunit]

Hi,

When I stopped for gas yesterday, there was a venting of air while unscrewing the filler cap.  My daughter, who had driven the car a couple of days earlier, noticed the same thing while stopping for fuel.  Is this normal for the car, or is there a problem than needs attention?

[Photon440]

I generally get a gust of built up pressure when I open my cap.  Since it`s a sealed cap, I wasn't surprised.