hey guys i went to put gas in my car the other day and when i unscrewed the cap it sounded like there was built up pressure inside my tank...it made a hissing sound and it sounded like i was relieving the pressure... why would there be any built up pressure in there?

 

More than likely it was actually sucking air in... The fuel system is closed loop... Say you fill your tank and there is an inch of air at the top... The in tank fuel pump pulls gas from the bottom of the tank and pushes it to the front of the car.. when it gets to the front it goes through your fuel filter then to your fuel rails... any fuel that gets there that is not injected into your engine gets pushed back to your tank... with nowhere for air to come into the system (because fumes would come out or fuel would evaporate out) as your fuel tank gets lower and lower that 1inch space at the top of your tank is becoming streached as your tank gets lower and lower... it essentually forms a low pressure area (less pressure than the outside air)... so when you open your gas cap air is puled into your tank to equilize pressure... Its normal tho... Dont worry about it

 

alright thanks alot man

 

Not a problem.... I quite enjoy geeking out sometimes... lol