Fuel Tank Vacuum?

Hi, this may have been asked before…but can't find it so here goes..

Sometimes when I take off my fuel cap to fill up there is a fairly big 'whoosh' noise as air is sucked into the tank - indicating that there is a vacuum created by pumping fuel out to feed my hungry engine!
Is this OK, or should the filler cap on GTI's be vented to allow air to be drawn in to replace the fuel?
I'm just a bit worried that the tank might collapse one day!
Anyone know for sure?

i was going to ask the opposite question. my old mk1 did the whoosh thing that your one does. she was a fit fast healthy old girl. the one i have now does not do the whoosh thing. i wondered if it should do it?

Been discussed many times before  :wink:

I believe the woosh is correct as the tank should be under a small amount of vaccuum - all the new cars I have have done it.  If there is no woosh then I suspect there is a problem with the filler neck or the breathers.

Cheers

Rich

I get the 'woosh' when I undo the cap when the tank is near empty.  I've had the same on previous vehicles as well.  
Shouldn't the breather hose equilise pressure in the fuel tank?

crazyquiff said

Been discussed many times before  :wink:

I believe the woosh is correct as the tank should be under a small amount of vaccuum - all the new cars I have have done it.  If there is no woosh then I suspect there is a problem with the filler neck or the breathers.

Cheers

Rich

how where do i check the breathers? my pump does not seem to prime before starting either. connected?

The pump priming is controlled by the relay and there seems to be two types -  primers and non primers

Not sure how to check the breather - it's the black cylinder type thing in the wheel well - been meaning to cut one open to see what is in there and map out the breather system.  If the breather is blocked then I guess you could get too much vac in the tank  8O

Cheers

Rich

All my cars have had the whoosh too but I think my sportline has a problem.

If I dont remove the cap slowly enough I can sometimes get fuel escaping too! When the car is less that 1/4 full I don't think its runs quite so well. Maybe this is a lift pump problem.
The cover under the arch used to me missing until I got a new one from crazyquiff (thanks Rich ) I could sometimes see fuel moving through the larger transparent breather pipe after stopping the engine and there has been a buzzing noise from trapped air moving about! When the car's tank is over 1/4 full I don't seem to get so many of these problems.

Also the fuel pump has started to prime twice when the ignition is turned on recently.

I know there is muck in the tank so I'm planning to strip the whole lot in the summer and see what I find. I've already had the lift pump out to check the strainer isn't blocked. The filler neck must have been replaced in the past but the tank not flushed.

I had problems with my lift pump on low fuel <1/4 tank - replaced it when it totally failed and problems were solved

Thanks y'all - I don't feel so worried now! The woosh thing is quite noticeable, especially when tank has been run from full down to nearly empty, which is usually what I do -can't see sense in filling up for a tenners worth every few miles!
I remember this being discussed years ago on VW magazine, but there seemed to be various views as to whether the cap should vent or not. Anyway, i shall continue to whoosh happily away!
While I'm on the subject..ish, is it possible/easy to alter the fuel gauge to read more accurately, as mine never goes to full, and shows completely empty with about two gallons still in the tank? I was wondering if the fuel sender/float could be adjusted, or even the gauge itself? - Anyone done this?

As fars as I can see cap should definetley not vent. All new/good caps appear to seal completely and are sprung to ensure this. In fact the same is true of the carb system. I assume that its the far greater pressure of the fuel injection system (60-80psi to 5-10psi for carb) that creates the vacuum.

The fuel float is on a bit of wire attached to the sender underneath the circular metal plate on the top of the tank. You could easily bend the wire to adjust the float, maybe it has been bent opout of shape in the past?

Cheers

Thanks Markwon,
will give it a look when I've got time…just when I was feeling smug about my nice reliable daily driver Mk 1, my brakes decided to go elsewhere…not easy to pump em when you need to stop in a hurry! Turned out to be a cracked brake pipe underneath, so there's my next job!!!
Oh joy!