Imploding Fuel tank

So, after about two years of the car not running (every few months for a few minutes), we picked her up from Germany last week. Drove to the nearest Dutch MOT/Customs stations to get new MOT and plates and after that another 250km home.
At the MOT station the tank was looking fine and straight. The next day at home she wouldn't start anymore (which was weird, because she has started on first turnover every time in the last five years)... Took about 30 turnovers and then she went. About two minutes later i heard a 'kloink' noise. I thought that it was my girlfriend loosening something on the roof. Until i looked under the car today, the whole fuel tank has buckled inwards! on top and bottom for a good inch or so! First thing that came to mind was a vacuum due to sludgy lines or so. Any thoughts on that?
The strange thing was that when i opened the fuel cap, there was absolutely no vacuum...

Has any of you had something like this before? And if so, could i form the tank back to it's shape with compressed air? But most of all, how to prevent in the future?

Before and after photos attached!

Blocked breather and or fuel filler cap breather hole. Or impact damage, a blocked breather will make starting difficult, try removing filler cap and starting engine.

flokkie-b said:

could i form the tank back to it's shape with compressed air?

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That would be a dangerous thing to do. The fuel tank is not a pressure vessel. Does it matter that it is dented? - live with it as it is.

Scooby22 said:

Blocked breather and or fuel filler cap breather hole. Or impact damage, a blocked breather will make starting difficult, try removing filler cap and starting engine.

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It is surely not impacts damage (as it's on top and bottom, uniform en no scratches) (and i didn't run over anything ).
As to my understanding, the breather just ends up under the filler cap and so only acts as a breather when filling up. A breather hole, i didn't find on the filler cap. And... there was no vacuum on the tank when i opened the cap...

Wimblowdriver said:

That would be a dangerous thing to do. The fuel tank is not a pressure vessel. Does it matter that it is dented? - live with it as it is.

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I do very well know it's not a pressure vessel, just thought that if the suction of the fuelpump is strong enough to buckle the tank, a bit of controlled pressure could easily press it back. Just to be sure everything in the tank is also straight.

flokkie-b said:

I do very well know it's not a pressure vessel, just thought that if the suction of the fuelpump is strong enough to buckle the tank, a bit of controlled pressure could easily press it back. Just to be sure everything in the tank is also straight.

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You have one snaffoo here as you mention there was no vacuum when you opened the fuel cap. Therefore implosion caused the seal to buckle releasing the vacuum.Also forcing compressed air into a petrol tank thats not cured will create fuel vapour from the broken seal. Water test and bunging the standard holes to search for leaks is normal.

Have you managed to get the filler cap off. It might spring back.

Happened to me on the boat once, 5 miles off sore, engine just died. Looked at the fuel tank and it was wrinkled like a prune. Unscrewed the car air rushed back in most of the wrinkles popped back out.

It is not the suction of the fuel pump that causes the buckling, it is the pressure of the atmosphere around the tank.

saxavordian said:

You have one snaffoo here as you mention there was no vacuum when you opened the fuel cap. Therefore implosion caused the seal to buckle releasing the vacuum.Also forcing compressed air into a petrol tank thats not cured will create fuel vapour from the broken seal. Water test and bunging the standard holes to search for leaks is normal.

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Hmm, that's a good one! Let's stay away from those vapes!
Which seal would you think? Filler cap, or somewhere on the tank itself?

tottot said:

It is not the suction of the fuel pump that causes the buckling, it is the pressure of the atmosphere around the tank.

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Well... it IS the suction of the fuel pump creating the negative pressure in the tank which the atmosphere around the tank is trying to even out by buckling the tank.

Mcclegend said:

Have you managed to get the filler cap off. It might spring back.

Happened to me on the boat once, 5 miles off sore, engine just died. Looked at the fuel tank and it was wrinkled like a prune. Unscrewed the car air rushed back in most of the wrinkles popped back out.

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I was hoping that was gonna happen, insteda no sis or his and the tank kept the same shape...

You can dismantle the fuel caps and clean out the breather.

flokkie-b said:

It is surely not impacts damage (as it's on top and bottom, uniform en no scratches) (and i didn't run over anything ).
As to my understanding, the breather just ends up under the filler cap and so only acts as a breather when filling up. A breather hole, i didn't find on the filler cap. And... there was no vacuum on the tank when i opened the cap...

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A fuel tank under vacuum if left for a while usually loses the vacuum gradually, so if you removed the cap the next day it would be at normal atmospheric pressure.

Just had another look at your photo, the bottom of the tank is double skinned, the outer skin being a bash plate which is not attached to the tank other than the mounting bolts.
If your tank had collapsed due to a vacuum the second skin would not have collapsed, I think there must be another explanation.

Rougharse Racing said:

Just had another look at your photo, the bottom of the tank is double skinned, the outer skin being a bash plate which is not attached to the tank other than the mounting bolts.
If your tank had collapsed due to a vacuum the second skin would not have collapsed, I think there must be another explanation.

View attachment 187143

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I don't think so, i just had a look under the car again and, as i didn't build it with one, there is no bash plate. That would also mean the drain plug is attached to the bash plate and wouldn't explain the buckling on top of the tank... So, i don't think that's the case.

Rougharse Racing said:

You can dismantle the fuel caps and clean out the breather.

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Already took it apart yesterday, but couldn't find a breather hole. That being said, the filler cap is brand new. This was the first trip outside of the garage!

Hmm, "Brand new" filler cap ? are you sure it has a breather in the cap (or elsewhere) to equalise the pressure as the fuel is consumed ?

marksman said:

Hmm, "Brand new" filler cap ? are you sure it has a breather in the cap (or elsewhere )to equalise the pressure as the fuel is consumed ?

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exactly that one: https://www.ebay.co.uk/itm/Locking-...fender-1987-99-200Tdi-300Tdi-V8-/283466083415
It has the little dent-out hole in the metal backplate, but i din't find any air channel leading to the great big outside world....

Hmm, I think i'll just unbolt it next week and re-check the whole system. Maybe even install an auxillary breather. Am not really waiting on this happening somewhere remote!

Thanks for thinking with me people!

It can't be a coincidence that you have fitted a new fuel cap and are now experiencing a vacuum build up in the tank.
From memory the breather is behind the disc held on with two screws inside the cap, however if the new cap is causing problems you may be better using parts of it to repair your old one.
I must say that I am impressed with the vacuum created by the fuel pump!