Why my petrol tank has pressure when i take the petrol cap off?
Answers:
Because of legislation, Modern cars have a vacuum system to stop petrol fumes escaping in the atmosphere. .
A certain amount of suction from the inlet manifold is used to create this condition.
What your hearing is the release of this partial vacuum whenever you take the petrol filler cap off.
Basically when your engine uses the petrol theres a vacuum and also the gas petrol creates is immense so when your tank is low there will be a certain degree of pressure and when you fill up you seem to be having to keep pressing the filler again and again but if you put the nozzle in and out a few times that should do the trick if not be patient and press the filler slowly and it should work.
Because the petrol expands when it gets warm.
The pressure doesn't come out it goes in.Drawing petrol through the pipe to the engine creates a vacuum in the tank that is released when you replace the vacuum with air.The noise you hear is air rushing in.
Because petrol caps these days are not vented. As the petrol in the tank goes down then the air pressure above it falls and you get a partial vacuum. As you take off the cap the air outside the tank rushes into the tank to equalise the pressure.
hi there,this is normal.there will be more suction the less petrol you have in ,am i right?this is probably becuase petrol is sucked from your tank for the engine.best practise is to always slowly unscrew your cap when filling.the fact that your tank is doing this shows that your petrol tank has a good seal too!.x
Petrol is an oil based product and lets off gas while in your tank. Obviously this gas has nowhere to go apart from out of the filler cap when you open it. Thats why.
Petrol is not just simply sucked out of the tank. There is a return pipe from the pressure system - injected cars - to stop fumes getting out and it is this that pressurises the tank.
RoyS
tank breather could be blocked . check underyour car at tank small pipe hanging from it can be full of dirt
this perfectly normal dont worry about it
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