When I fill up with fuel, there is often a loud hissing noise, as if you are opening a bottle of fizzy drink or something whenever I unscrew the petrol cap.

It also seems like there are vapours rising from the hole, as there is a shimmering shadow from the tank.

What is the cause of this?

Is this a problem?

Does this mean my fuel tank ventilation valve is broken?

I have a 2003 Opel Agila

  • I'm pretty sure all this is normal. I've noticed the same on every vehicle I've driven. I don't know exactly what causes it, though. Commented Sep 29, 2015 at 20:16

1 Answer 1


Regulations limit the amount of unburnt hydrocarbons that can be released into the atmosphere, therefore fuel tanks on cars now have to be sealed to stop these emissions. The noise you hear is air rushing into the fuel tank, due to the low pressure caused by the use of fuel.

  • Shouldn't air be allowed in and not out? Couldn't the hiss be the air in the tank coming out after expanding due to the heat of driving? Something has to let air into the tank, right?
    – JPhi1618
    Commented Oct 9, 2015 at 20:17
  • @JPhi1618 The ventilation system does let air in when a vacuum is pulled in the tank. Vapours that are captured in the charcoal canister while standing are sucked into the engine while driving. So it makes sense that when you pull up at a fuel station you will have used fuel, so the pressure in the tank will be slightly lower than outside the tank, therefore air will be pulled in when you take the cap of.
    – HandyHowie
    Commented Oct 9, 2015 at 21:11
  • How does using up fuel reduce the pressure within the gas tank? Assuming that temperature remains constant as you use up the fuel, there will be an increase in the ullage space which will allow existing vapours to occupy more volume and thereby reducing the pressure. But vapor pressure is a function of the temperature always. Since fuel is at a constant temperature, so the remaining fuel within the tank will evaporate to maintain the vapor pressure at that temperature. That hissing sound should be, I guess vapors at higher pressure escaping into atmosphere from tank rather than vice versa. Commented Dec 6, 2022 at 10:56

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