Pressurised Fuel Tank - How/why

Fumes are kept in for environment protection, shaking fuel makes it try to expand and the fumes get pressurised, all this is extremely normal.

mrcheerful

Hi. Thanks for the info. I'm not worried about it, just interested in how things work. Cheers, Steve

The message from "SandS" contains these words:

Are you sure it's pressurised? They're often slightly evacuated, which makes a whoosing noise as you open the cap.

There is a vacuum only when there is a fault.

Some cars have a sensor and if the tank does NOT pressurise, a fault light comes on. First question to customer with fault light: Have you got a fuel cap?, second question: Did you do it up till it clicked three times?

mrcheerful

The message from "mrcheerful ." contains these words:

Really? All the Corsas I had did it, as does my Montego and the wife's Clio.

I haven't yet tried the Audi, mostly 'cos I'm putting off the awful day when I have to fill it up. Depending where I get the diesel, a full tank will be around £70!

yes, the caps and breather systems are designed to let air in to replace the fuel as it is used. in the absence of proper breathing the tank can collapse or the fuel flow can stop.

mrcheerful

The message from "mrcheerful ." contains these words:

True, but the air admission valve always seems need a psi or two to open. When the tank's low, 1psi deficit is quite a bit of volume - hence the whooshing noise.

All the cars I've seen try to keep a slight vacuum. The newer ones that try to detect leaks do that with vacuum too. Vacuum is free, you just get it from the inlet manifold. Pressure is a bit more difficult to come by. Also, if there is a small leak, air will leak in with a vacuum rather than fuel/vapour leaking out.

IIRC the regs my Saab was meant to comply with allow anything up to a

Cheers,

Colin.