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MG Midget and Sprite Technical - Fuel Tank Collapse!!

Hello

I was just stripping down the rear suspension to change the springs today and noticed that the petrol tank on my 1500 looks like it has been sucked in. Like there has been a huge vacuum in it and it collapsed inwards. It doesn't leak.

Does anyone have any experience of this?

I put a new petrol cap on it and did notice a lot of pressure when it was released but didn't think it was that much.

I guess I need a new tank?

Any advice would be great thanks.

Simon
Simon Taylor

Simon. It sounds like either you have a North American specification car or the home market cars used the same fuel tanks the NA specification did. With the NA spec cars, the fuel filler cap was tightly sealed and the fuel tank was vented to the charcoal canister in the engine compartment with a vapor seperator in the trunk connecting the the line from the tank vent to the line going forwards. If any part of that system becomes blocked, the tank does not vent, the fuel pump creates a suction in the tank, and the thin metal of the tank is deformed inwards, releasing the vacuum when the fuel filler cap is released. I have seen this happen a single time.

I suggest you find why the tank is not venting and correct that problem before deciding if you need to purchase a new fuel tank. Without the problem being solved, a new tank would deform just as the old tank has.

Les
Les Bengtson

was the previous cap vented and this one not?

non-tech now, I've not got a physics/petrolchemical degree !! - petrol expands with the heat and contracts again when cool

so filling the tank and leaving the boot end at least in the sun could pop the tank back out but then of course you can't remove the cap again until everything is cool, night time perhaps

then of course you need to fit the correct cap (if this was the cause of the problem)

regardless of how much the car is use the paper copy owners Handbook is your friend

(Ref: 0058) - http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html

plus you can back it up with the less convenient but more comprehensive DVD - http://www.motoringclassics.co.uk/heritage-range/c380
N Atkins

Simon,

I've heard of it happening before with diesels where the tank breather has become blocked but not on a petrol. Your experience shows it's possible with petrol also. What fuel pump are you using?. It may be possible to fix the tank by pressurising it to reverse the problem but it depends on the extent of the damage, in any case if I were to try it I would use water to apply the pressure and as the maximum pressure that could cause the damage would be about 14.7 psi you might easily get away with mains water pressure to re-inflate the tank. Any chance of a picture.

I don't know whether the 1500 had a petrol cap with a breather or not but you need to make sure whatever breathing was intended that it isn't blocked.
David Billington

I don't think I would choose to use mains water pressure to "re-inflate" the tank. Adding water is unnecessarily introducing another problem.

If you can seal the filler cap, you could try a compressed air supply via the fuel outlet pipe. In theory you could even pump it up using a tyre foot pump. You could try connecting a short length of rubber hose to the outlet , with a tyre schrader valve fitted into it, maybe held with wire or Jubiilee clips.

Guy
Guy

I'm not sure compressing an air/fuel mixture is a good idea.
Jeremy Cogman

Guy I think there is a major benefit of using a liquid (water) rather than a gas (air)

With air you end up with a pressurized container than could explode with expanding gas. If it was filled with water then because water will not compress after the tank has expanded the pressure disappears and the tank becomes safe.

Do we believe that the tank has been sucked in? or has it been dented from the outside?
Robert (Bob) Midget Turbo

Hmm, yes perhaps you are right! I wasn't imagining putting very much pressure in but certainly the air/fuel mixture wouldn't be so clever if a spark happened to get added to the mix!

Better delete that offering, and substitute silence!
Guy

Simon
Is the bottom of the tank concave ? I believe thats quite common on spridgets, simply caused by the tank being grounded and dented upwards, or jacked under the fuel tank.
Just trying to suggest alternatives.
Ian
Ian Webb '73 GAN5

Guy,

My suggestion of using water was 2 fold, it is incompressible so doesn't store energy when used for a pressure test or similar unlike air hence its use for testing pressure vessels, and mains water pressure is often much higher than atmospheric 14.7 psi . So the failure mode of a hydraulic test is relatively benign compared to the failure mode of a pressure test using air or some other pressurised gas as a working medium.
David Billington

Yes David,
I have already withdrawn my suggestion.
Guy

Hello All

Thanks so much for helping me. It now appears that I have used a US spec petrol cap.

The petrol tanks is quite concaved but doesn't appear to be leaking at all. At the moment I can't afford to buy a new petrol tank so am wondering if it is ok to drive really until I can.

I am thinking it won't just spring back but you never know!

I can't upload a pic at the mo as the car is not at my house. Next time I go over I will take a picture.

Thanks

Simon

Simon Taylor

Pressure in the tank should expand it; too much pressure might give you an extra gallon or two. Pressurized petrol and compressed air is called a bomb.

You might blow it out by removing the cap at night, letting it cool, and refitting the sealed cap in the morning so it is sealed as the sun heats things up. Put a plug in the outlet pipe from the tank to do this.

Not that hard to drain the tank and fill it with water, then you can use water or air pressure. If there are locking ridges at the edges of the collapse, tapping them gently with a mallet whilst under pressure usually lets it pop out. I've fixed a number of them like this,. both vacuum and ground dragger dents.

FRM
FR Millmore

Simon see my previous post - I'm surprised that the other chaps aren't at least giving this simple route a try first but they see things on a more technical level that is very useful should my suggestion not work

having on many ocaasional run my tanks very low I've had them suck in and pop back out again a few times, how much the inplosions cause the tank to concave, other than once, I don't really know

now half jokingly I say, the more you use your car the more you'll learn and understand want is and isn't serious

fabulous weather now drive it and get it in the sun to resolve your problem whilst at the same time enjoying driving your car
N Atkins

apologises to FRM I was still typing whilst he posted and I missed his post

he's also suggesting you try the simple method
N Atkins

put a vented cap on, ignore the tank, drive and enjoy :-)
David Smith

This thread was discussed between 02/07/2011 and 03/07/2011

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