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This is new to me, as is the car, as is petrol rather than diesel ... but each time I visit the pumps and open the filler cap a brief rush of air escapes, almost as if I'm releasing a vacuum. Never happened to my diesel cars.

Naturally I've googled it and I'm getting mixed opinions. Some say that it's a natural phenomenon as the fuel system is pressurised and some cars have a 'fuel tank vapour system' that produces this effect.

Others seem to think that the escape of air is the sign of a fault as the tank should be able to breath this vapour out and something might therefore be blocked.

Does anyone else experience this and/or can it be explained? I'm sure that my experience is not unique and I'd really like to know the real reason behind my fuel tank gently farting at me when I go to fill it!


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From Google SD....

Why does my gas cap hiss when I take it off?
The hiss is a normal phenomenon, ideally keep the tanks at more than 1/2 full to keep the fume formation and vapor loss at check. Hiss means your fuel tank vapor system is working properly and not releasing harmful fumes out.22 May 2016
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Yep, thanks for that John, I remember googling that too ... but the alternative (my 3rd para) view is still of interest as clearly not everybody is in agreement. 😉

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39 minutes ago, Sundance said:

fuel tank gently farting at me when I go to fill it!

As john above says this is perfectly normal as the car has a sealed fuel system.

Be thankful Toyota/Lexus did not continue with the system fitted to the American version of the gen2 Prius. This has a bladder in the fuel tank to hold the fuel, and in certain circumstances farts the fuel back out of the filler tube while being filled. Also if the bladder leaks into the tank for any reason the tank assembly has to be completely replaced.


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3 hours ago, Britprius said:

 and in certain circumstances farts the fuel back out of the filler tube while being filled

A fart with liquid follow through is not a fart!

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I have the same hiss of air escaping when I open the tank lid, never had a chance to ask in here.

Never had anything like that on diesels as you mentioned above (never though about it before you mentioned it). Good to know the system works then! 🙂


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This is what the manual for the IS300h says


When refueling the vehicle
Observe the following precautions while refueling the vehicle. Failure to do so may
result in death or serious injury.
● After exiting the vehicle and before opening the fuel door, touch an unpainted metal
surface to discharge any static electricity. It is important to discharge static electricity
before refueling because sparks resulting from static electricity can cause fuel vapors
to ignite while refueling.
● Always hold the grips on the fuel tank cap and turn it slowly to remove it.
A whooshing sound may be heard when the fuel tank cap is loosened. Wait until the
sound cannot be heard before fully removing the cap. In hot weather, pressurized fuel
may spray out the filler neck and cause injury.
● Do not allow anyone that has not discharged static electricity from their body to come
close to an open fuel tank.

I thought I remembered it said something about the "whooshing". I didn't remember the warnings about static electricity, so that's useful to be reminded. Now I just need to work out where there's some unpainted metal.

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