# Fuel tank query.



## Tagbartok (Mar 30, 2013)

Hi.
When I remove the fuel cap to fill up with petrol there is a hissing sound caused by air pressure releasing. I'm assuming there is a vacuum forming in the tank as the fuel level drops. Is this normal or is there a blocked breather somewhere?
I had this on VW Beetle back in the 1970s and it was a blocked breather causing fuel starvation and the engine to cut out.
On the TT though it does not seem to cause a problem.
Cheers,
Steve


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## nilrem (Sep 9, 2011)

Tagbartok said:


> Hi.
> When I remove the fuel cap to fill up with petrol there is a hissing sound caused by air pressure releasing. I'm assuming there is a vacuum forming in the tank as the fuel level drops. Is this normal or is there a blocked breather somewhere?
> I had this on VW Beetle back in the 1970s and it was a blocked breather causing fuel starvation and the engine to cut out.
> On the TT though it does not seem to cause a problem.
> ...


iirc I read some where that this can be caused by the evap (charcoal) canister under the bonnet on the lhs by the coolant expansion bottle.

If the pipes have been removed in the past and put back in the wrong position (switched over) it can cause the same symptoms as you describe.

hth


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## Tagbartok (Mar 30, 2013)

Thanks Nilrem.
I've looking for a diagram to show which pipes go where if anyone can help please?
Cheers,
Steve


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## nilrem (Sep 9, 2011)

Have you scanned the car at all ?

I've done a bit of digging and there's a valve N80 attached to one of the pipes.

The purpose of the valve is to open and allow the petrol fumes under vacuum to be drawn back in and be burnt.

It could be your valve isn't working and the excess petrol fumes are being released when you open the cap.










Numbers 1&2 are Evap


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## Tagbartok (Mar 30, 2013)

Hi nilrem.
Thank you for the diagram. Where did it come from? I have bought a copy of ELSA but I'm struggling with it but that's another story!
No I haven't had the car scanned as I've only just considered the vacuum to be a problem.
Strangely there doesn't seem to be any other problems arising such as engine roughness or cutting out and there is no fault light showing on the dash. 
I've been digging too and found horror stories of tanks collapsing and fuel pump power connections shorting out and causing sparks in the fuel tank with disastrous consequences. I'm tempted to change the relatively inexpensive N80 valve as a matter of course but that could be the beginning of the slippery slope of changing bits to see if it will cure the problem. Not the most economical way to fix something I know. 
In the meantime I have a long drive down to Bagshot tomorrow morning so I've slackened off the fuel cap to hopefully reduce the vacuum.
I will get the engine codes read next week if I can wrest the car from my wife. 
Cheers,
Steve


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## Hoggy (May 8, 2002)

Hi, This may help..


No. 5 is vent line from evap canister to purge reg valve.
No. 18 is Vent line to Evap canister.
No.19 is vent line from evap canister to purge reg valve.

Hoggy.


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## nilrem (Sep 9, 2011)

Hoggy said:


> Hi, This may help..
> 
> 
> No. 5 is vent line from evap canister to purge reg valve.
> ...


Nice one Hoggy that's a way better pic than I posted


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## Tagbartok (Mar 30, 2013)

The TT performed faultlessly on the 160 mile motorway trip yesterday morning. I did slacken the fuel cap to reduce the vacuum. When I get back I'll test the N80 valve properly. 
Cheers, 
Steve


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## Tagbartok (Mar 30, 2013)

Hi. Hopefully I have now fixed this after much searching for similar issues but been left with a feeling of exasperation! This is quite a long post which raises a couple more questions so please be patient.
I found this thread on the VW Audi forum dating back some time

http://www.vwaudiforum.co.uk/forum/show ... e-build-up

Crasher had the solution. The simple colour coded push on connectors (on what I think is a fuel accumulator located below and to the right of the expansion vessel) had been mixed up. This to my mind is a bit like a qualified electrician wiring an electrical plug up wrong. What is more simple than black to black, white to white and blue to blue yet the grease monkey who refitted them, probably when the cambelt was changed, managed to mix the blue and the white up!
This now makes me question the competence of any work that has been done on the car and proves why I like to do things myself.

I am a tad concerned about what effect this mix up may had elsewhere. When I released the connectors about 50cc of fuel came out of the white connector pipe. I know the unit I've called an accumulator isn't the charcoal filter because that's located further forward of the expansion vessel. Does anyone know what this unit is? It has four pipes connected to it and the frontmost one goes to the charcoal filter and what effect the pipe mix up may have had?

I haven't driven the car yet to see if the pressure issue has gone so fingers crossed.

Cheers,
Steve


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## Tagbartok (Mar 30, 2013)

I've just driven 170 miles and the pressure in the tank has gone since I refitted the pipes correctly...RESULT!  
Steve


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## nilrem (Sep 9, 2011)

So the pipes were reversed as I suggested but not on the evap canister ?


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## Tagbartok (Mar 30, 2013)

nilrem said:


> Tagbartok said:
> 
> 
> > Hi.
> ...


hth[/quote]


nilrem said:


> So the pipes were reversed as I suggested but not on the evap canister ?


Hi nilrem.

Whether it was the evap canister or not doesn't matter as you you were indeed right about the pipes. It wasn't until I found out that the pipes were colour coded that I realized mine had been fitted incorrectly though.
I would be interested to know what the device is we are talking about and if any other any damage could have been caused to the charcoal flter? 
Another consequence of the fix has also arisen. The LED fuel consumption display is now working properly??? Before I switched the pipes over parts of the LED numbers were missing making the display almost unreadable. Now they are perfect.
Cheers,
Steve


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