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Discussion Starter #1
When I unscrew the filler cap there's a hissing sound which is either caused by a vacuum in the tank (and the air is rushing in) or pressure in the tank (air is rushing out). I can't tell which it is.

Is this normal or do I have a blocked breather somewhere? :surprise:

The car is performing perfectly, but I don't want to strain the fuel pump.

Anyone else have this?
 

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Earth Dreams :)
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927 Posts
Tanks these days are plastic, therefore Flex outwards when filled up with a liquid.. as this is consumed and you release the pressure, the hissing is the tank popping out a lil effectively, thus a small release of pressure.. I think.
Mine has the hapless system ans does it when i put the nozzle into the tank apature..

Normals me thinks, nothing to be worried about.
 

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Clutch pedal guru
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5,098 Posts
It's actually the air rushing into the tank from the less than atmospheric pressure created by the fuel pump in the tank. This is so to release LESS hydrocarbons into the atmosphere.
 

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Premium Member
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3,216 Posts
It's a vaccum. Air tight tank, as petrol is pumped out, you get low pressure that's then released when you open the cap. As others have said, it's air rushing in rather than out. Probably done for environmental and safety reasons. A tank full of petrol can become pressurised on a hot day due to the vaporisation. If you naturally have a vacuum in there from usage, it gives more capacity and results in less pressure. Tanks didn't used to be like this but then again they were probably heavier and thicker than the tanks of today. So probably, environment, safety, construction cost.
 

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Ex-Pat Geordie!
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2,267 Posts
Mine is also capless so don't get around to the filler to notice if it happens or not? Always noticed it in other cars I've owned though.
 
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