Has your fuel tank got a vacuum in it?

[newsh]

When you go to fill up does air rush in when you unscrew the filler cap?

[Tonks4x4]

Yes what is that all about? Is the breather blocked or something????

[twoqu]

Me too! I was going to start a thread myself!

[Nuvo]

I took this to be excess of fuel vapour being evacuated to the atmosphere. When I think it should be fed back to the carbon cannister and converted to clean air and then into the IM for mixing?

I expect there must be a pressure relief valve on or connected to the fuel tank is no longer able to open.

[SteveH]

I've always found there's some negative pressure in the tank, possibly due to the vapour recovery system drawing fuel fumes out of the tank. If there was a serious issue/blockage the tank would collapse under atmospheric pressure?

[jade]

Mine started doing this also about 6 mouths ago

[ericd]

the emptier the tank the more air rushes in surely ?

[shark_90]

No I don't get this at all on my Avant.

[S2central.net]

I've never had it.... and I don't think you should get it... the EVAP system is designed to have th tank vent properly... It should neither pressurise nor vacuum AFAIK. Suspect blocked pipe to the carbon canister or an EVAP solenoid that isn't behaving properly - may still be connected and keep a fault code away. Do some output tests on VAG-COM to check the EVAP valve is at least responding.

[lini]

If the EVAP is vorking properly then the valve opens at loo idle so off boost. that causes cca. 0,5bar underpresure in the gas tank. there is a nonreturn valve for the EVAP system that prevents boost coming into the tank. There is only one more exit from the tank and that is filling cap. That one should have an presure relief walve for sunny days and a valve to let the air in but it has to be with a pretencion otherwise there is no sense to have that system. There is no way that air could go into the tank beside EVAP system. So always when you drive of boost motronic opens that blue ******* and sucks the fuel fumes from the tank and that causes underpressure in the tank. If my theory is right no underpressure in the tank can only show a broken filler cap.

[S2central.net]

I have some Bosch info that conflicts with that Lini... I'm not 100% sure it is specifically accurate for S2 - its a generic description of the EVAP system.

Duty cycle of the EVAP is very low at idle - reason being that if it vents too much fuel vapour into the IM while idling that it can upset the mixture adaptation.
EVAP is also shut off on the overrun (as are the injectors) to prevent unwanted fuel vapour getting into the CATs.

[newsh]

Interesting, not all the same.
My car always had some vacuum in the tank, I figured not surprising as the valve clicks when the throttle closes sucking the fumes through the carbon can. When the fuel was running low the vacuum would make a groaning noise.. But now my carbon can is gone to make way for the catch tank and the tube from the fuel tank is capped off. I now have much more vac in the tank than before, like it wooshes in for a couple of seconds when you undo the cap.

[S2central.net]

If you have deleted the carbon canister, you need to let the tank vent to atmosphere... Surely if you uncap the vent tube where the carbon can was that you'll have no more problems with the tank getting vacuumed. Should really be done with care and consideration for a what-if the car lands upside down in a hedge one day - some sort of 1-way ball valve or whatever to stop fuel pishing out the breather would be handy.

In what way did you delete the EVAP solenoid down under the TB hose then ?

[newsh]

If you have deleted the carbon canister, you need to let the tank vent to atmosphere... Surely if you uncap the vent tube where the carbon can was that you'll have no more problems with the tank getting vacuumed. Should really be done with care and consideration for a what-if the car lands upside down in a hedge one day - some sort of 1-way ball valve or whatever to stop fuel pishing out the breather would be handy.

In what way did you delete the EVAP solenoid down under the TB hose then ?

I agree, I need to unplug the pipe from the tank, just wondering what to do with it, I know others have vented into the rear wheel arch etc..

IIRC there's a ball valve at the tank end anyway. Solenoid is still there and connected electrically but not connected to any pipework, that spigot on the IM is capped off

Paul, the plumbing to the carbon can is done with crimps on the rubber hose connections to the to copper (or steel) line from the tank, these were quite loose on mine and may be the reason you're not getting solid vacuum. Either that or the fuel cap has gone wrong in some way.

[Davidsinclair05]

I am sure that Audi said that the tank was ment to have a sealed cap.
This happened all the time on my 3B. If you put on a vented cap the car suffers from poor fuel economy and stalls.