Pressure in fuel tank?

[iwbus]

I have a 1994 Escort estate and when I remove the fuel cap I seem to get a small blast of high pressure air. The missus used to have an xr2i and says she had the same problem with that. Any idea what causes it? Is it normal?

[andy hughes]

Most cars do this. your tank seal is air tight, and once you seal it, the vapours released natrually as the petrol sloshes around are released, but unable to escape, so they collect in the air space above the fuel. Once you open the air tight seal, the vapours escape, with that telltale whoosing sound. No problem.

[Potatoman]

Nope, as the tank drains, a vacuum is formed in the tank. Its air getting INTO the tank. Dont matter tho, its harmless enough. How dangerous would it be if petrol fumes shot out of tank evrytime they were opened?

[Bob...]

Nope sorry both wrong. Most fuel systems work on a slightly positive pressure. More fuel is pumped to the engine than it can use and what is't used is returned to the tank causing the pressure, this helps to push the fuel around the system as well. That said the fuel tank is vented by a valve/filter if needed in case of over pressure otherwise the tank could blow up like a baloon and split. The fuel tank could not work on a vacuum because at best fuel would stop getting to the engine as the vacuum increases as the the fuel is used and at worst will in fact collaspe the tank. The sound heard is indeed the pressure being released and is quite normal.

[Symon]

This depends on particular car and its particular fuel system. Some are fully vented, some are slightly negative. I have known examples of tanks being considerably distorted by negative pressure when vent blocked. I have not heard of designed positive pressure within fuel tank of normal production vehicle. There has been for many years a fuel cut off device on on oulet from tank which operates in crashes to prevent fuel spilling from fuel lines near hot components. Inagine the possible consequences if tank pressurised even slightly and then ruptured in an accident.

[keltik]

my 1999 jimny has a negative pressure which is a pain, if a drop of water is near the cap when i remove it, its sucked into the lip between cap and tank. i cant really see how there would be a significant pressure on the fuel return.