Plums

Everytime I remove the fuel filler cap I feel and hear that I'm
braking a vaccum seal in the petrol tank.......

The further I drive between fill-ups the worse it gets.

Is this normal or could the breather pipe blocked?

This also leads me to think that it's harder for the engine to
pull petrol through.... causing a possible reduction in power?

The car is MY01 13000 kms

Cheers for any help!

Hanslow

I think this is normal. My MY00 does it too. I'm sure I've read somewhere that it should do this.

Just checked my Handbook and it mentions that the fuel in the tank is under pressure and to be careful undoing the petrol cap in case it flies out

I guess it works on the same principles as drinking out of a big coke bottle without letting air in. When you take your mouth off, all the air rushes back in to equalise pressure.

Don't think it's anything to worry about anyway

Plums

Thanks for the rapid response!

O.K. but what possible benefit can it have?

I have noticed the harder I drive the car the stronger the
vaccum....

I have also seen the warning about petrol spilling out
but if a vaccum is created then air is trying to get in on
the opening of the filler cap......
Wouldn't this make it impossible for petrol spill out?

Hanslow

I'm only guessing, but it may well keep the petrol down where it is supposed to be so it doesn't slosh about as much. As for the petrol spilling out, maybe the influx of air might push some of the petrol out?

I'm only guessing so don't take any of this as gospel I've just never worried about it. Does it on my missus' Puma as well.

mashwrx

are you sure it's a vacuum ? I can understand pressure due to heat expansion etc, but vacuum does not sound right to me. I think modern cars have a re circulating 'sealed' fuel system, where by the return feed from the engine goes back to the tank, aids in cooling the fuel and no leaks to atmosphere and all that emission stuff...just a thought..

Hanslow

Just had a quick search on google and I think you are right mash. The tank is sealed (from what I can find) and the hiss is actually petrol vapour escaping. It has nowhere to go until you open the cap. This would explain why you get more hiss after leaving longer intervals between filling up.

Sound plausible?

Plums

Maybe but I can actually feel the suction when removing the cap!

If it was pressure trying to get out then it would push into my hand.... which isn't the case.

Maybe the anti slosh theory is true?

Cosie Convert

The filler cap has a little relief valve in it. Not sure if it lets air in or vapour out though.(stuck maybe ?) The vapour normally goes back to the engine via that black canister in the offside inner wing.

ustolemyname??stevieturbo

iTrader: (1)

It is more than likely a little bit of pressure in the tank. Because these fumes arent generally allowed to escape to atmos, at certain times while driving, they are vented into the inlet manifold thru a silly charcoal canister filter thing at the front of the car. Operated via a solenoid controlled by the ecu. You will find it ir pressure, because if you remove the fuel feed line to the filter/fuel rail the pressure will pump small amounts of fuel out. Releasing the pressure by opening the fuel filler will stop this happening. If it was vacuum no fuel would spill out.
A completely useless answer for u all!!!!!!!
im sure nearly all subs do it!!!!!!