5th Gen Fuel Tank Help

stal1878 · Post by **stal1878** » Tue Sep 04, 2012 7:25 pm

Hot sure where to put this thread

Hello chaps.
Every time I remove my filler cap to fill her up it sucks in loads of air. Can anyone tell me as to why? My theory is that the tank breather is blocked and when the engine takes away the fuel there isn't anything to replace it with, hence the vacuum, make sense?
If this is the case, can anyone direct me towards how to solve it? Was thinking about drilling a hole in the filler cap, but would like a more professional fix if at all possible.

Cheers fellas.

P.S. Does anyone know what the bulb for the reverse lamp is called, its a really fat capless one.

Post by **Pushki** » Tue Sep 04, 2012 7:27 pm

Mine does this too. It's normal!!
I don't know the reason for this, but I'm sure it's something logical.

Post by **Donald** » Tue Sep 04, 2012 7:35 pm

Mine does the same thing.

SCHHHHHRRRRRRRRRR it says.

Post by **Vtecmec** » Tue Sep 04, 2012 8:01 pm

Yes all my cars have done this, the tank is a sealed system, I suspect that it occurs due to the condensation of gasses, creating a slight negative pressure. I notice it's more prevalent when the tank is less full, and therefore full of more gas vapour.

stal1878 · Post by **stal1878** » Tue Sep 04, 2012 8:02 pm

Hmmm, I would have thought the constant expanding and contracting would have weakened and split the tank...

Suppose it could be "normal" 3 seems a bit more than change?
IIRC the door does say "Remove Slowly" as well......

stal1878 · Post by **stal1878** » Tue Sep 04, 2012 8:02 pm

I only ever open my cap when its very empty so I suppose I only see it at its "worsed"

Post by **RattyMcClelland** » Tue Sep 04, 2012 8:48 pm

Guys the fuel system is under a vaccumm pressure as fuel gets sucked through the fuel lines.

Diesels are much lower. My van doesnt do it as strong. The low the fuel in the tank the more air gets sucked in.

stal1878 · Post by **stal1878** » Tue Sep 04, 2012 8:55 pm

Surely the fuel is sucked out of the tank and a breather should let air back it so the tank doesn't warp?

I've had a Fourtrak, Cherokee, 328i, 330Ci and drive works 323i, Astra and Ranger and they don't do it.
I know on the E38 if it does it then you need to unblock it before it rips the tank(thats steel though).

Anyway, so long as its "normal" and isn't going to rupture my tank I don't care

Post by **RattyMcClelland** » Tue Sep 04, 2012 9:56 pm

Yes your right. It's not a true vaccumm as such but its still under high pressure. Your relieving the pressure basically. More fuel in tank also means more fuel pressure which is good. But then it does weight more.

vti2003 · Post by **vti2003** » Tue Sep 04, 2012 9:59 pm

It is because it is a sealed system and there is expansion and contraction due to the temperature variations as well as pressure due to the mechanics of the system itself. Can be a vacuum or pressure from temperature rise/drop or a mix of all of them.

Unplug the fuel line from your fuel rail on a cold morning and see how much fuel pours out by the time the day has warmed up.

If you don't take the fuel cap off the system pressurizes itself as the temperature rises- learnt that one the hard way