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Ok so iv'e been tearing my hear out trying to find the cause of my bad fuel consumption.
There is also a fuelly smell around the rear of the car- fillerneck side so i dropped the tank and had all the lines off.
The top of the tank was a bit damp when i took it off. I then ran out of light so left the tank in the car for a couple of days- when i came back it was really damp.. There was hardly any fuel in the tank by the way.
The strange thing is, the wetness didn't smell of fuel. Does that mean it's not a leak? Or would the smelly part of the petrol evaporate off leaving odourless liquid?
Any advice would be much appreciated.
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I just did the same thing with my tank last week for the same reason. I found this:  The underside is pretty much pristine under several layers of underseal, as is the inside of it. Puzzling over it while spending a few hours with the wire brush getting that lot back to bare metal, I think what might be happening is this: In the mornings, the cold petrol in the tank saps a lot of heat out of the air as it's volatile and wants to evaporate (maybe more so now the supermarkets are blending so much ethanol into theirs). That in turn causes a lot of dew out of the air to sit on the top which pools and rots it. That wouldn't explain why it got more damp inside the car, other than perhaps you looked at it earlier in the morning before it had chance to dry out a bit? |
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Interesting theory.. that would certainly explain it.
If that was the case you would expect everyone to have damp patches on top of their tanks i suppose?
Anyway in the absence of a better explanation i'll go with that, although it would be interesting to know if this is normal. Particularly since we both have bad MPG and a fuel smell at the tank.. Did you ever solve either of those problems?
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Damp patch on fuel tankBenzBoy
@benzboy
Club Retro Rides Member 7
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I think Calamity's explanation rings true - the space above the fuel inside the tank holds moisture-rich air which condenses during temperature changes. Over enough time it'll rot the tank at the top. I had a Mk3 Escort which had exactly that problem - I only found out when I brimmed the tank in preparation for a long journey!  |
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I've only just refitted the tank (after an acid etch, zinc primer, tough black topcoat and a good squirt of waxoyl) so I'll let you know once I've gone for a run.
As there's no leaks evident, I'm hoping the petrol smell was down to the solder sealing one of the vent tubes having curled up a bit. All I've done is resolder it with plenty of flux. There's a much stronger petrol smell up front from the fuel in the carb bowl boiling off after a run. There's only an inch or so gap between it and the exhaust header.
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