Excessive Pressure in fuel tank

[VR6Pete 0]

Well, my MOT was a bit of a flop today!

 

Few issues (expect other threads to appear :) )

 

but one issue is that it stank of fuel... from the tank area... no leaks, has been bone dry, but after having it running a large build up of pressure is seen in the tank, and when i un-screwed the petrol cap, there was a massive hiss and it stunk of fuel! so seems to be a build up of pressure...

 

some advise needed as to what could cause this, and what can I do to resolve the issue...are there any common issues / breather pipes etc to look at?

 

Cheers

 

Pete

[owen g60 10]

Possibly the valve from the carbon canister is causing it.

 

Disconnect the tank return from the valve an it should relieve the pressure

[VR6Pete 0]

Thanks i will take a look at that... to see if it is functional.

[Kevin Bacon 5]

Yep. Sounds like the tank vent is blocked.

[VR6Pete 0]

Is the tank vent the same as the valve thing? Or something else? :)

 

---------- Post added at 10:07 PM ---------- Previous post was at 9:39 PM ----------

 

found this photo on the webs - is it anything I need to check thats shown in this pic?

 

http://www.use.com/showorig.pl?set=e47bea2b895a3ca1d513&p=3

[VR6Pete 0]

Yep. Sounds like the tank vent is blocked.

 

Where is the actual tank vent mate?

[VR6Pete 0]

Possibly the valve from the carbon canister is causing it.

 

Disconnect the tank return from the valve an it should relieve the pressure

 

Is that the valve that is mounted to the strut behind where the air intake is ? Any way of testing that valve works? I unplugged the cable and ran vagcom and it threw an error, plugged it back in and it went away? :)

[RW1 0]

Is that the valve that is mounted to the strut behind where the air intake is ? Any way of testing that valve works? I unplugged the cable and ran vagcom and it threw an error, plugged it back in and it went away? :)

 

Yes, the blue unit on the strut tower forward face.

 

You can check its operation in VCDS using VCDS Output Tests - 03. You hear a faint "tick,... tick,...tick" coming from it if its responding to the engine ECU commands. http://the-corrado.net/showthread.php?16110-VR6-Misfire/page8&highlight=output+test+injector

 

Also check you have not got a pinched pipe on the engine bay to tank run (clear hard tube) and also the pipes to the carbon canister have not been pinched in some way.

 

If none of the above, then back to the tank filler. There is an anti-gravity valve at the top of the filler neck inside the wheel arch. (Prevents fuel escaping if the Corrado flips in an accident). They can seize closed so the tank doesn't breath. Its on the front side (ie.headlight end) of the filler neck top and looks like a table tennis ball with a pipe to it (breather to the charcoal filter). Part number 357 201 751. A clip holds it in which slides at an upwards angle to release it. In vertical normal postion the gavity valve breaths, tilt to 45 degrees either way and it shuts.

 

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Edited May 5, 2012 by RW1

[VR6Pete 0]

Some good info there, will have get vag com out check valve... Checked around the filler neck and I see the below...

 

Right side....

 

Left side...

 

 

Now. There was a massive build up on mud and all sorts around that area... Where exactly am I looking for the breather can't see anything like a ball shape?

[RW1 0]

Did it from memory but have edited my post since.

 

First photo, right side. You can see the black breather pipe and rusty clip from the charcoal filter on top of the gravity valve. The breather pipe is attached to the spout coming off the top. Theres a sliding clip in that remaining mud that releases the valve.

 

I was describing it from underneath in the wheel arch as a ball well you llok upwards which is the bottom half you can see in the photo.

 

.

Edited May 5, 2012 by RW1

[daleyboy 0]

Cant help with that, but if you want to find a big patch of rust just drop the tank and look behind the neck just behind where your photo shows.....as you say a magnet for mud......

[VR6Pete 0]

Did it from memory but have edited my post since.

 

First photo, right side. You can see the black breather pipe and rusty clip from the charcoal filter on top of the gravity valve. The breather pipe is attached to the spout coming off the top. Theres a sliding clip in that remaining mud that releases the valve.

 

I was describing it from underneath in the wheel arch as a ball well you llok upwards which is the bottom half you can see in the photo.

 

.

 

Thanks for that info. I think I'll start there as its suspect :) does the slide clip just slide upwards? And out it pops? I'm hoping maybe I can clean it up or something... Is is possible to remove this without having to drop the tank?

[RW1 0]

I would check the engine bay charcoal filter valve with VCDS Output Tests first and be sure there are no pipe kinks as thats easier. If nothing amiss found then tackle the gravity valve.

 

The gravity fuel valve sits on a round rubber seal (N90359601) inside the main white plastic body. You may find that is perished when removing the valve.

 

There is a flat blade type clip (rusty) on gravity valve top the far right in the first photo. Pull upwards to enable the gravity valve to be released. Its not listed as a part and may well come with a new gravity valve.

 

Hence why you should tackle in the order posted as parts may be hard to source, ie. the gravity valve securing clip not listed by VW.

 

.

[VR6Pete 0]

Hi everyone.

 

Did the output tests...

Could hear the valve going tick tock...

 

On this one couldn't hear anything, is this an expected output?

 

So I moved onto removing the gravity valve, removed all operating as I'd expect it too....

 

I then blew down the black pipe from the gravity valve, couldn't blow down it, is this expected?

 

I then removed the clear pipe on the charcoal filter and tried blow down that, couldn't blow down it, couldn't feel anything coming out of it either or smell of fuel vapour etc... Is this expected? Had a look for kinks and nothing is visible...

[RW1 0]

On this one couldn't hear anything, is this an expected output?

Not fitted. So correct with only N80 tick-tocking and can assume the valve is operational.

 

I then blew down the black pipe from the gravity valve, couldn't blow down it, is this expected?

That includes the charcoal filter! Makes it a little harder!! Clear pipe has a straight unhindered run to the back of the car.

 

I then removed the clear pipe on the charcoal filter and tried blow down that, couldn't blow down it, couldn't feel anything coming out of it either or smell of fuel vapour etc... Is this expected?

That runs to the tank gravity valve and turns into a black pipe at the gravity valve. With the black pipe disconnected from the gravity valve, it will be hard to blow but on expelling your lungs, hold it and see if the pressure you've built up quickly dissipates. If not, there's a blockage - obviously.

 

My clear pipe always stinks of fuel in the engine bay end. Lack of fuel smell suggests a blockage.

 

Take the black pipe off the gravity valve and substitute a short piece of piping to stop wheel arch dirt getting in. Run the car and check the tank doesn't pressurise. It shouldn't if your gravity valve check is correct.

 

Next bit means dropping the fuel tank as you want to get at the point behind the tank where the clear pipe from the front of the car joins the black pipe upto the gravity valve. To check and isolate the blockage into which section, black or clear pipe.

 

.

[VR6Pete 0]

thanks for the reply...

 

it does look as if its suggesting a blocked pipe, or trapped somewhere... does anyone know the route of the pipe?

ive been looking at many fuel tank photos on the net, but no luck seeing where it runs.. would i be able to see any of the pipe if i removed the plate in the boot above the fuel tank for a closer look?

 

dont really want to drop the fuel tank if I really really have too :)

[VR6Pete 0]

All sorted now. Looks like someone in the past had not routed the black pipe from the gravity valve properly and was crushed againt the underside of the car. Pipe now has shape and is properly routed, I can now easily blow through the pipe and now getting fumes going into the evap canister. Many thanks for your help.

 

Pete