Gas Cap releases negative pressure when opening

[FranGuy]

Hi all,

Title basically says it all but when I refuel after burning a large amount of gas the tank seems to have a vacuum. I have a 1994 e420. I had this happen on a 1990s 4Runner I used to own and never worried about it, but I saw another thread on this forum that indicated that this could be a problem. Is this a problem? When I think about it, it does seem like there could be a one way valve that releases pressure that is blocked. And this could be putting excess load on the fuel pump. Any advice is much appreciated!

If this is the wrong category to post please let me know and I’ll move it!

Thanks

 

[gsxr]

Slight pressure in the gas tank is normal. When removing the gas cap, pressurized air may escape with a "whoosh" sound.

However, if there is VACUUM, that is bad. If there was vacuum, you'd have to pry the cap off the filler neck after loosening it. Vacuum can cause the tank to collapse and require replacement. The fix for this is usually replacing the plastic vent valve below the gas tank, on the driver side. Details here.

 

[FranGuy]

Oh interesting… I thought the woosh was from the air going in when I break the seal. I have never had to pry the cap off so maybe it’s positive pressure… I’ll check next time I fill.

Thanks as always!