Intermittent Fuel Tank Leak Mk2 Scirocco

[craigwalton]

Hello All, hopefully someone can shed some light on my fuel tank leak issue on my 1992 Mk2 Scirocco Scala which has done 87,000 miles. Last week, there was a leak on the right hand side of the tank when I filled the tank to the brim. Turned out to be a missing hose clip between the filler neck breather hose and the top of the tank. All sorted - see diagram.

image002.png

Edit: Leak was at filler neck breather hose (fat green pipe) connection (not gravity valve breather hose)


This week, a new leak has developed, appearing to come from the top rear left hand side of the tank, collecting on the tank's flange then down onto the axle or exhaust heat shield at about a drop per second. This was discovered after a 10 min drive to get my wheels aligned. I drove it back home where the leak continued at that rate for about 20 minutes. I dropped the tank about an inch (until it hits the rear axle). I put in an additional 5l of fuel which got me comfortably over the amount of fuel there was when the leak sprung - no leak. I tried blowing on the breather hose at the top of the tank in the hope I'd hear air escaping from the tank - nothing. I ran the car at idle for 15 minutes with no heavy leak but did produce a damp, rusty patch right above the flange directly above the exhaust pipe - see photo. Note the paint "bubbling" in various areas.

IMG_2547.jpg

Why was it dripping heavily after I'd driven the car, then not leaking at all with the car off, then just damp with the car running at idle? Could it be related to heat or vibrations from the exhaust?

I've checked the pump + fuel lines accessible from the back seats and they're dry. I've tried Petro Patch putty on the area where the rust was visible, but I'm not confident about the root cause of the leak. I'm considering buying a new tank, but would like to understand more about the problem before I do. Thanks in advance for any answers or suggestions!

[the edmundator]

I suspect you've got rust pinholes in the tank. Not sure why you'd get the variations in how it leaks, but rust in the tank is quite common, often just presenting with a vague whiff of petrol round the rear of the car.

I would buy a new tank and check the filler-neck (off the car) at the same time - you might find you need both.

[Mark H]

Same issue as I have, only found out I had a leak when I brimmed it for the Nats Meet and found I had a weep, Ran it down to just over half full, no leak.
Best place I found for a tank was Classic VW £96..

[DannyH]

If it's an injection they build pressure in the tank which might account for the difference.

I'm not very technical so wait for someone more knowledgeable to give you a better answer.

[scrappy1382]

as previously stated by mr edmundator - strip it all down and replace as much as you can for new . pain in the bum, but worth the reassurance of knowing its all been replaced

[the edmundator]

mr edmundator

I didn't spend five years at (a now-defunct) medical school to be called "Mr."

The lady from Playschool signed my degree certificate, for goodness' sake!

[scrappy1382]

[craigwalton]

Well thanks very much for the responses, chaps! The consensus seems to be order a new tank, which is what I'm going to do - I'll check out Classic VW. Until the new tank arrives, I've wire brushed away rust and loose paint, sanded, cleaned & degreased before slapping on some petro patch putty and in less accessible areas, some chemical metal.

IMG_2561.jpg

Once the new tank arrives and I've got the old tank out, I'll inspect the fuel lines and the filler neck to see if they need replacing.

I'm going to do some tests just for my own sanity on the leaky tank once it's on a bench. Still doesn't add up how I got a heavy leak for 30 mins then (before any repairs were done) dampness upon idling the car with more fuel in the tank and finally me blowing into the tank to increase pressure. I should add that I believe the car has been off road for nearly a decade before I owned it and I'm not sure if the tank was brimmed - could be internal corrosion? If so, could the fuel build up between the tank and the paint, causing bubbling and eventually breaking the paint off? To me, although the outside of the tank is brown in areas, it does not look particularly rusty.

Does anyone happen to know the difference between fuel tank part numbers:
155 201 075 B and 155 201 075 C? ETKA lists the C-spec as ""J..""? I understand that the part numbers 171 201 075 P and 171 201 075 Q are for the 40l tanks as opposed to the 55l I'm looking for. Thanks again!

[DannyH]

from experience make sure you keep the fuel in the tank low before you remove it!

[mark1gls]

I would clean the threads and start soaking the rear beam bolt threads in some penetrating oil as you will need to remove the rear beam to get the tank out, when undoing the nuts losen a little then tieghten back up then losen a little more and do back up and so on.... This will clean the threads and lessen the chance of them shearing off as its a right pain to replace the studs.
Also check the bottom nut and bolt on the rear shocks as they may need be be removed but if they are rusted in place you can undo the shocks at the top but it's a right fiddle getting them back into place when you come to put the beam back on.
Brake hose joints may also benefit from a clean and spray with penetrating oil.
WD40 is not very good as a penetrating oil I use plugas, you can buy it from Eurocarparts.

[craigwalton]

Thanks for the removal advice Danny & Mark! I managed to loosen the rear axle bolts on the LHS as well as the brake line where it turns from rigid to rubber connection on the LHS. I have not yet managed to loosen the strut's bolt/nut onto the axle. If unsuccessful at the next attempt, I'll buy some of that oil. If that doesn't work, I might look into disconnecting them from the chassis within the boot, as you suggested? I've ordered a new tank from Classic WV for £96 - should be here by the end of the month!

[craigwalton]

New tank fitted, running fine. As promised, I've taken a close look at the old leaky tank to try and learn a thing or two:

1. Outside of tank is in relatively good nick - few patches of missing paint with rust behind, especially on the top of the joining flange, presumably where standing water can gather.


2. I bunged all 4 holes and filled tank with water. No obvious leaks.

3. Took a dremel to the tank (leaving water in there to expel all petrol fumes) and cut out an inspection hatch:
-The top surface of the tank is as shiny as new with some small surface rust patches which polished out with some steel wool.

-There was at least a table spoon's worth of sand like rust sloshing around (unsure if this originates from the tank or maybe an old filler neck, as the current one seems new).

-The lower half of the tank is consistently rusty, but does not seem to be 'deep' rust - some steel wool eventually reveals a shiny surface.

- The 2 radii which converge into the join of the tank are sometimes free from rust for some reason.


4. Took some sections out to show the join of the tank:

- first section is from an area which looked rusty on the inside.

- second section is from the area which appeared to be leaking, which I had applied a fixing patch to (I chiselled some of this off).


- the section has a crack right though it. It certainly wasn't this bad when it was on the car, but may still have been present.



Questions:
Are the 5 fixing points to the car all rigid relative to each other, or is it possible that as the car is driven and has forces exerted on it, could cause the tank to stretch and flex slightly? These forces could cause fatigue cracks in areas.
Could this be exacerbated due to the proximity to the exhaust?

Thanks again for everyone's advice!