Air Coming From Petrol Cap

[springbulb81]

When I go to fill up my car, when I unscrew the petrol cap I feel quite a strong release of air come out.

Is this normal? Something I need to get looked at?

[grasscarp]

Normal

[gness]

All my cars have done that, Spring....some more than others for some reason, but I have never worried about it.....☺

[SeaJayPea]

Are you sure it's coming out? More likely to be going in, surely.

[SeaJayPea]

Snap.

[grasscarp]

The gas tank builds up pressure because the air that is fed in to replace the fluid gets hotter during operation. Wait until the car has cooled off and you will not see nearly as much pressure.

[bhg481]

I thought the fuel system was supposed to be vented. If it weren't, as you used up the fuel, the vacuum created would eventually stop the fuel being pumpeed to the engine. It could also make it extremely difficult to get the fuel cap off.

[springbulb81]

No it's definitely coming out. Unmistakable

[=rizla=]

BHG, i had that problem on an allagro years ago, broken cap so it created a vacuum, after a long journey, it was a nightmare to get the cap off, and you could hear the fueltank 'pop' back into shape.

It never had any running problems, although with that car how could you tell!!!!

I solved it in the end, i left the cap at a petrol station, then replaced it next time i filled up and noticed with these universal ones.

[TWR]

You have no problem Spring, as said It's the normal occurance, there should be a vent hole within the petrol cap.

[IWillHelpYa]

Hello springbulb81,
What you hear escaping from your filler cap is not air, it is the vapour produced by the petrol in your tank. As you drive around, the petrol is being agitated inside of the fuel tank. Once the fuel is agitated it will produce vapour, this vapour will build up slight pressure within the fuel tank. The only time you will become aware of this is when you open the filler cap and the vapour pressure is released to the atmosphere.
Do not worry about this happening as it is considered normal. It does however prove you have a good seal on you vehicle's filler cap, which will prevent rainwater entering the fuel tank. All is well.

[dl56uk]

I've just posted the same Q? so it must be just a normal thing if you drive days on end and only open the cap when you need fuel