I heard air going trough the fuel cap... is that normal?

Question

I have an Audi 1998 1.8T Quattro.
I have a Evap leak somewhere. I check a lot of hoses under the hood and around the Leak Detection Pump and I have found no leak. But recently I change my Gas Cap hoping it was the problem and I've seen no change. One day I walk around while the car was running and I heard air going trough the fuel cap. So I replace with the old cap and it was a bit less but still air going trough. So i was wondering if it was normal or should it be no air to create some pression ???

Answer 1

This concept of creating pressure in the fuel tank has me baffled. The only pressure I know of is of gas that has evaporated from the liquid state. (experiment....if you carefully poured some(lets say a cup) gas into a gal container and then put the screw cap on tight and shook the containter for 5 seconds, when you opened the container you would hear a whoosh sound. That is the gasoline as a gas vapor escaping.
With the air thru the cap...as liquids heat up thru the day more evaporation is happening creating that hiss. You don't want it to leak out as much as you want air to leak in.(compare that to drinking a bottle of Coke and never taking your lips off but sucking continuously) It won't come out unless there is something to replace the vacuum left behind.
Still, with all that being said the fuel cap should seal air tight. There are lines that go back to the engine compartment to allow the air into the gas tank. You possibly have a cracked or ill fitting hose that you are overlooking.

Answer 2

the sound that you heard is from feul being used and not having any way of releasing the pressure. But you still should not have the sound so pronounced. Check for damage around the fuel intake there may be a small bend or crack.In any case there is little chance of lost to evaporation.

Answer 3

This is normal. Your fuel tank will always have a positive vaccum while the engine is running. Your fuel cap is designed to relieve this vaccum so that fuel flow is not interrupted or your tank isn't damaged.