Hi all.

I have a mk4 fiesta 1.25, and every time I go to fill up with petrol, there is a build up of air in the tank, it releases every time I take the cap off.

Just wondering weather it is normal or not?

Cheers, Jason

 

are you sure its not sucking in when you take the cap off? as ive had it when the tank breather gets blocked

 

I'm not sure, it does sound like its built up air pressure being released when I take the cap off, how would I know if its sucking air in?
Also, the filler cap becomes hard to unlock, every time I take it off, though sometimes its not as bad as other times.

Thanks.

 

stick your hand round it when you take the cap off and try to feel what way the air rushes.

when my tank breather got blocked on my fiesta si i filled the tank up and when the fuel got low it cut out due to an air lock in the tank lol. when i took the cap off the fuel tank made a funny noise where it was poping back to shape after imploding lol

 

It's not air it's vacuum. It's the sound of air rushing in to fill the void. When the fuel is pumped out of the tank it is usually replaced by air that is drawn in through the breather. When the breather gets blocked air can't get in the tank to replace the volume of fuel that has been pumped out.

Think of you drinking from a bottle of coke. If you don't take your mouth away from the bottle to let air back in, the bottle starts to collapse in on itself as you drink more and more from the bottle. The same happens to the tank but the tank is a lot more rigid in construction so fights the vacuum more.

 

Thanks for alll your replies.

So how do I check, clean or replace the fuel breather?
Where is it located?

Cheers, Jason

 

Anyone?

 

I'm no expert, but always assumed the breather is the small hose that runs parralel with the filler pipe.....?

 

Are the fuel caps not vented? When I removed the tank out of my mk4 fiesta I dont recall seeing a breather.

 

Doesn't the vent exit back into the filler pipe, just by the top of the filler neck? (Well above the tank level)?

 

Wish I hadn't binned my tank out of mine I could of checked.

 

THE filler pipe to the tank is split into 2 parts so it can be filled more efficientlyso the air comming out of the tank can escape fore easily rathe than come through the petrol you are pumping in! not sure about breather though. my wifes 1.25 mk 4 used to do the same and wee never had any probs!

 

Every single fiesta I've come across has done this.

 

Older cars had a small vent hole in the filler cap. After that, they had a vent pipe that vented underneath the back of the car. They then went to a vent pipe from the tank (or filler neck) cannister in the engine bay to absorb the vent gasses, which is then purged into the intake system when the car is started. I think they introduced the cannister system about the time cats became standard, so around 1993.