Petrol tank hissing

Honest John Back Room » Technical matters
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Tue 20 Jan 2004 14:39
Petrol tank hissing - Old Codger
When I refuel my petrol car unscrewing the petrol cap releases a loud 'suction hissing'. Is this ok and why is it happening?
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Tue 20 Jan 2004 14:54
Petrol tank hissing - rory
If your car is also an old codger no air is getting into its fuel tank. Left untreated ( ie top not unscrewed - and it will get progressively harder to get off ) the tank can implode because fuel going to the carb(s) will leave a vacuum behind. For very old cars drill a small hole on the side of the filler neck near the top;this lets air be drawn in and problen solved. Foe newer cars, probably a breather system problem which will be a variation on the foregoing.
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Tue 20 Jan 2004 21:28
Petrol tank hissing - Civic8
Sorry to correct you.But modern fuel systems are sealed so if you take off fuel cap and it sucks in air all is well.

>>For very old cars drill a small hole on the side of the filler neck near the top;<<
You don`t do that on any car.If a breather problem occurs find out why and replace the offending breather pipe or unblock it.
never drill a hole in the filler pipe or any other petrol pipe.
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Wed 21 Jan 2004 09:37
Petrol tank hissing - rory
Well, yes. ok. However, the car it happened to me ( and my mates, since it was a joint purchase costing £10 back in 1966 ) was a Ford Pop, and we couldn't even see any breather pipe !
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Tue 20 Jan 2004 15:31
Petrol tank hissing - Civic8
What car and age is it.
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Tue 20 Jan 2004 21:45
Petrol tank hissing - Ivor E Tower
Call the dealer or importer's technical helpline - some cars are supposed to do this. I had a Honda which did this so I called Honda UK and they told me that it was quite normal as the fuel system was supposed to operate under a partial pressure. I kept the car for nearly 18 years, it always did this and the fuel tank did not start to collapse, so no "breather" problems.
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Tue 20 Jan 2004 21:51
Petrol tank hissing - Blue {P}
My 03 plate Fiesta always hisses when I remove the filler cap, as did the last one on an 02 plate, so I probably wouldn't worry. :-)

Blue
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Wed 21 Jan 2004 00:35
Petrol tank hissing - Dynamic Dave
Are you sure the air is getting sucked in and not being blown out when you remove the cap? Some tanks slightly pressurise due to the venting system letting air in, but not back out again - also when the tank is low of petrol you'll get a lot more vapour build up.
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Wed 21 Jan 2004 10:28
Petrol tank hissing - AN Other
I'd agree with the "hissing is normal" brigade. It must be something to do with pressurised fuel systems, but every modern car I've driven over the past 3-4 years has done this, and they weren't all knackered!
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