Hiss of Air When Fuel Tank Opened

Honest John Back Room » Technical matters
View Threaded
Sun 15 Jun 2003 20:36
Hiss of Air When Fuel Tank Opened - timp
When I open the fuel tank of my Focus to fill it up, I always hear (and feel) a significant hiss of air around the filler cap. Is this normal?

I thought there was a fuel tank air vent which would let air into the tank as petrol is used, to avoid a vacuum in the tank. Or is the purpose of the fuel tank air vent to let air out of the tank when petrol is put in?

Hope this isn't a stupid question...
Tim
Reply | Report as offensive | Link
Sun 15 Jun 2003 21:30
Hiss of Air When Fuel Tank Opened - leatherpatches
My Civic does that as well. Don't think it's a problem and if there is something letting air in, it probably does so slowly. Thus if you have been using fuel (by driving to the petrol station), it won't have had time to equalise. I suppose a good check would be to open it when you haven't been driving for a while...in fact, I'll try it now...

...no hiss whatsoever. It ALWAYS hisses when I fill up, so I think my theory is correct. You have no worries, pal.
Reply | Report as offensive | Link
Sun 15 Jun 2003 22:01
Hiss of Air When Fuel Tank Opened - Andrew-T
If you 'feel ' a hiss of 'air' when opening your filler cap, surely it is blowing, not sucking? Injection engines have a closed fuel system where unused fuel is returned to the tank. Fuel added at the pump will normally be cool because of underground storage, as would your car first thing in the morning. After some miles driving, the car is warming up, and warmed fuel has been recycled to the tank, pressure will normally rise, causing your hiss.
Reply | Report as offensive | Link
Sun 15 Jun 2003 23:34
Hiss of Air When Fuel Tank Opened - Blue {P}
All three of my last Fords did exactly the same, with the first 2 I wondered if something was wrong, by the third I thought it must be meant to do that, and now I know it is!

Between them I've clocked up nearly 20,000 miles and never had a related problem. I wouldn't worry 'bout it :-)

Blue
Reply | Report as offensive | Link
Mon 16 Jun 2003 09:20
Hiss of Air When Fuel Tank Opened - Garethj
Cat-equipped cars have a pressurised fuel system (about 2 - 3 psi) to stop harmful fumes escaping when the car sits in the sun.

The cap has a pressure valve in it so if you lose the cap always replace it with the correct one to maintain the pressure setting.

Gareth
Reply | Report as offensive | Link
Mon 16 Jun 2003 17:15
Hiss of Air When Fuel Tank Opened - timp
Thanks to all for your replies. I haven't experienced this on other cars, so I thought I'd better check!
Tim
Reply | Report as offensive | Link

Looking for a garage you can trust? Honest John's Good Garage Guide has thousands of reader recommended garages, plus how-to guides and tips on servicing.

The Good Garage Guide

If you cover no more than 100 miles per day and have easy access to a charging point at home or work, then an EV is ideal for you.

Is an electric car right for me?