Vacuum in gas tank

[Dan353]

Hi,   This may be a dumb question.   If I drive my car for about 1/2 hour to hour  my gas tank seems to build up a lot of vacuum does anyone else have this?  My gas cap is stamped vented and anti surge. Is the vent just for expansion?  Just trying to figure out if my gas cap is bad. 

Thanks   Dan 

[OldGuy]

The vent should serve to both relieve pressure as well as vacuum. I'm surprised that your fuel pump will still function with the negative pressure that you are experiencing. At some point (negative pressure), it will cease to function and your engine will probably die.

Frank

[68blk500c]

Many years ago, as I assisted a friend fire up a fresh 289 just dropped into the car, he had installed a new (then Ford) gas tank (and a little gas), but left the plastic filler neck hole plug in place for the time being.  The Ford fuel pump pulled enough vacuum to buckle the bottom of the new tank.  That fuel pump continued to work fine later.

[GT350DAVE]

On a long trip back from a SAAC convention buckled a fuel tank. The car had a new repro pop open cap. The seal on the cap must have been overly effective. It pulled the bottom of the tank all the way up to the fuel pickup so naturally the car starved for fuel and stopped.
Dave

[Royce Peterson]

Actually the vented cap only allows air into the tank. It does not vent fumes outwards, if it does it is defective.

Your tank should have an outward woosh of fumes / air when the cap is removed. Be sure that any time you remove the cap you don't have a lit cigar in your mouth!

I am betting there is no vacuum. If there was the car wouldn't run. It sounds normal and correct.