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Petrol tank breather
- Chazg83
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- Chazg83
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15 Feb 2014 16:00 #104600
by facade
If it suddenly breaks, go back to the last thing that you did before it broke and start looking there
Replied by facade on topic Petrol tank breather
Depends on what you mean by a breather.
When the car is standing and the weather is warm (like it used to be in The Olden Days before it rained all the time) petrol evaporates. as more evaporates the pressure in the tank rises, until the one way valve in the top of the tank opens and allows the overpressure vapour to escape to the evap cannister, which absorbs the fumes. When you start the engine the evap cannister is vented to the manifold to suck the fumes into the engine.
(if you overfill the tank, the evap fills with liquid fuel and the car has a little trouble running until it clears the excess fuel)
As the engine runs fuel is used up and the tank starts to build up a negative pressure, and would eventually squash flat, so there is a vacuum relief valve in the filler cap that opens at a reasonable pressure difference.
At the MOT, the tester undoes the cap and listens for a shhhhhh as air rushes in to prove the cap is sealed.
The tank breather line goes from the top of the tank to the top of the filler hose, as the bottom of the hose is full of fuel, and you would never be able to fill the tank up if air can't get out of the top.
When the car is standing and the weather is warm (like it used to be in The Olden Days before it rained all the time) petrol evaporates. as more evaporates the pressure in the tank rises, until the one way valve in the top of the tank opens and allows the overpressure vapour to escape to the evap cannister, which absorbs the fumes. When you start the engine the evap cannister is vented to the manifold to suck the fumes into the engine.
(if you overfill the tank, the evap fills with liquid fuel and the car has a little trouble running until it clears the excess fuel)
As the engine runs fuel is used up and the tank starts to build up a negative pressure, and would eventually squash flat, so there is a vacuum relief valve in the filler cap that opens at a reasonable pressure difference.
At the MOT, the tester undoes the cap and listens for a shhhhhh as air rushes in to prove the cap is sealed.
The tank breather line goes from the top of the tank to the top of the filler hose, as the bottom of the hose is full of fuel, and you would never be able to fill the tank up if air can't get out of the top.
If it suddenly breaks, go back to the last thing that you did before it broke and start looking there
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- markyp2000
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15 Feb 2014 18:38 - 15 Feb 2014 18:41 #104613
by markyp2000
Replied by markyp2000 on topic Petrol tank breather
At the mot the tester ie me dont listen for a shhhh it has nothing to do with the test all we test on the cap is the rubber seal dont matter is it shhhh or not
the tank has a air return pipe at the top of the tank to the top of the filler neck to let air out as you fill
iv never hurd a evap canistor filling with fuel seeing as thay are fitted higher than the tank
Your tank is sealed as long as the cap has a good seal trust me mine been sitting in water
At the pic posted of the fuel tank above there is no vent to air as its against the law that also why engines have brather pipes running to the intake system so it all gets burnt off
on moden cars
the tank has a air return pipe at the top of the tank to the top of the filler neck to let air out as you fill
iv never hurd a evap canistor filling with fuel seeing as thay are fitted higher than the tank
Your tank is sealed as long as the cap has a good seal trust me mine been sitting in water
At the pic posted of the fuel tank above there is no vent to air as its against the law that also why engines have brather pipes running to the intake system so it all gets burnt off
on moden cars
Last edit: 15 Feb 2014 18:41 by markyp2000.
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- Jutwit
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