Sucking noise from filler cap

When the engines been running a while I get a hissing or sucking noise from the filler cap, could it be a blocked vent? Is it easy to get at to clean? I suspect it might be causing running probs

Cheers

Do you mean the fuel filler? If so perhaps the seal has gone on it? Mine hisses air in or out when I remove it.

Kirkynut

Yup the fuel filler, seems to hiss all the time, is there a breather for the tank? Just looking round google looks like there's a valve in the filler cap?

Cheers

There is a one way valve in the cap, that allows air into the tank, to prevent the tank sucking flat. It should prevent fuel coming out of the cap if the car falls over. I've never noticed mine hissing, but I don't stand next to the filler when it is running.



The fuel system is sealed, to prevent fumes reaching the atmosphere.

When it gets warm, and the fuel expands, it fills up the expansion space in the tank and there is a pressure valve in the small vent line that opens and allows vapour into the charcoal cannister next to the alternator.
The cannister is vented via a short rubber pipe to the air, and the charcoal holds the fuel vapour.

Next time the engine starts, air is drawn in through the charcoal cannister to the inlet manifold, purging all the vapour back to the engine.


If you overfill the tank, by shaking the car and filling right up the spout, on a hot day liquid fuel gets in the charcoal cannister, and this does cause a problem at the next start up when the cannister is purged.

I always wondered what the canister was for. Thanks

It is called an EVAP cannister, the purging is via an electrically controlled valve on top of the manifold, I'm not 100% sure when that valve opens, must be at tickover though, as there won't be enough vacuum to pull air through the cannister otherwise.


Actually, thinking about it, if you gently blow into the vent line at the cannister end, you can pressurise the fuel tank, (and check for the "hiss" as you crack the cap open that means you have no leaks) so the vent line valve in the top of the tank must be a one way affair with a pressure relief, and air could get in that way, through the evap cannister) to vent the tank if needed.