Has your fuel tank got a vacuum in it?

[macspring]

Not me too,

Always drive from tank full to tank empty, doesn't take long, most days, lol.

But anyway, my 95 ABY Avant never "whooshes" when the filler cap is unscrewed.

Car is standard as regards it's fuel system.

My Mk 1 Golf GTi used to make the said noise.

Posted just for comparison purposes.

HTH somewhat.

Kind regards.

[newsh]

the emptier the tank the more air rushes in surely ?

Eric, yours is a ABY, does it WHOOSH?

[Pirvu Eugen]

I had an colapsed fuel tank because the valve wich is suposed to let the vapours go to the carbon canister but the liquid no... has blocked in closed position... and after an 600km nonstop trip.... my tank had a capacity off 7 liters
Imagine my face in front off the gas station..... with 1.2 liters of fuel at 100 km...
Good fuel consumtion no?
Now the carbon canister is out and the tank vented to atmosfere.... Captain Planet is after me.

[newsh]

Paul and those that don't get a whoosh, do you get a smell of fuel in the car ever?

I usually do for a while after filling up.

I'm wondering if the vacuum is causing cavitation in the fuel giving me an occasional misfire coming back on the throttle after a neutral throttle, say when going down hill in traffic then going to speed up a little..

[newsh]

Tank will be vented in about 5 mins!

I had an colapsed fuel tank because the valve wich is suposed to let the vapours go to the carbon canister but the liquid no... has blocked in closed position... and after an 600km nonstop trip.... my tank had a capacity off 7 liters
Imagine my face in front off the gas station..... with 1.2 liters of fuel at 100 km...
Good fuel consumtion no?
Now the carbon canister is out and the tank vented to atmosfere.... Captain Planet is after me.

[4WheelDrifter]

Mine used to whoosh before I did away with the carbon canister.

Tank will be vented in about 5 mins!

Just vent it out the air vent in the rear wheel well, I sure there are pictures on here somewhere. I've also heard of cars running lean due to a problem with the vent hose. I hope you also blocked the line to the IM?

[Pirvu Eugen]

The strange thing is that the fuel cap has only pressure relief valve. How the air is coming in to compensate the fuel consumed by the engine + the vapours sucked in the carbon canister?

[S2central.net]

This is weird... I definitely get no fuel smell. I definitely get no 'whoosh'. AFAIK - that is correct behaviour. I think we need a poll on this one !

[s2driveruk]

yep always had this on my 3b and is usually more audible on low fuel and hot days. I was never sure if it was vac or pressure though...

[Error404]

Every coupe i have had has done this, as well as ever Jap car i've had.... AFAIK its negative pressure in the tank - air getting sucked in. Doesn't seem to do any harm - as long as the tank is vented i guess.

When I put my fuel tank back in i'm putting a breather behind the rear bumper and deleting the carbon crap - one more thing to go wrong and gets rid of loads of useless junk in the engine bay.

[driftin jo]

My friends A4 started cutting out every now and then , the problem turned out to be a valve that was sticking causing large vacuum in the tank, the Audi garage said it was supposed to just hold neutral pressure ie. the same as current atmospheric minus a small percentage about 25 mb or so.
On the car in question they showed him it on a ramp you could see the tank start to collapse when the engine was running , so much so that it would cut out because of lack of fuel pressure.
I would have thought that the s2 worked in a similar way ie you should not get a noticeable rush of air in when you undo the filler cap.
My S2 does not do this by the way and no smell of petrol.

[newsh]

Well my car has not done the mystery misfire since venting the vac line. The line had a piece of tube on the end which used to go to the carbon canister, this was plugged with a bolt and pipe clipped. The piece of tube now has a hole in it and the clips have been loosened. I went for a 10 min drive and no whoosh when I released the filler cap. Will check it on a longer drive. I'll re route the vent out the back at some stage I guess.

[S2central.net]

I'm convinced here after thinking more about it - the whoosh is wrong - the whoosh is bad.

If the woosh is air rushing in - then the tank is under some signicant vacuum - which can't be good... A tiny little vacuum is to be expected by means of the EVAP function - but thats an enviro feature to prevent fuel tanks venting vapours - especially on hot days when pressure in tank rises. Taking vacuum on the tank to extreme will start to collapse it and reduce its capacity.

If the woosh is air rushing out - then the tank contains pressurised air - thats not good either for obvious reasons. Expanding the tank with extremes of air pressure have nastier potential side effects.

For anyone else that has the woosh when removing the fuel tank >>>>

Disconnect the pipe between carbon canister and where it goes into the EVAP solenoid. Bung the engine side of this hose and let the carbon canister side vent to atmosphere. Then after some miles driving you should have no woosh. I'll see what Bosch info I can dig out here, but it seems to me that in order for the tank to vent fumes into the IM under EVAP control and NOT build up vacuum that there must another means of venting pressure. There might even be a vent to ambient in the carbon canister itself for this.

[newsh]

Paul, it was definitely air rushing in, always has been, but when the hose was plugged it was serious vacuum.

Incidentally, when he was mapping the car the other day Mihnea said he was surprised by how lean the engine was running at small throttle openings..

[jade]

I have the bottom port blocked with no problems

Early this year i started having problem filling the tank (it keeped cutting the pump out)

A little after i found the plug unattached from the carbon canister plugged it back in no more problems.

A while back i remember seeing a thread by bos2 saying if you fit a solenoid valve to the bottom of the carbon canister connected to the throttle sensor to open it, it will still function correctly.

Anyone have info on this ???