Hi Guys,

I got a call from my father today saying that oil was dripping from my trunk (1960 Plymouth Belvedere). When I got back to the car there was a largeish puddle of oily type stuff under the tank. The tank is not original, it is believed to be from a Cadillac, age or type unknown.
The car is well protected with underbody seal. Also, it is quite a hot day today (Well, for the UK, about 26c).

The stuff appears to be coming from the back of the tank, I'm guessing it is the underbody seal. There appears to be no sign of fuel having dripped down from the filler, and the tank seems very solid, so would be surprised if there was porosity. The leak would appear to be along the rear lower edge of the tank, but seems pretty solid.

I opened the petrol cap and got a pint of fuel spat at me. I'm guessing fuel expanded in the heat. Shouldn't there be a vent on the tank that prevents pressure from building up? I'm guessing that this would be on top of the tank. Doe these ever become clogged or stop working? Is it an easy fix?

Any ideas you may have as to why this may be happening would be greatly appreciated.

Thanks alot,
60Ply

My gut says a non vent style of fuel cap, plus heat, caused the fuel to expand. Eventually something gave, maybe the gasket at the top where the sender is mounted. The pressured fuel sprayed the undercarriage, disolving the undercoating which is why you see black and oily. To add to it, gas dissolved blacktop very very well, so the oil in blacktop of driveway, IF APPLICABLE, would contribute to the oily puddle you see underneath.

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1976 Dart Lite /6 3 Speed

Backtobasics has much of the right idea in mind, and I agree with him that the "oil" your car is leaking is in fact petrol-dissolved underseal, but your car is meant to have a non-vented petrol cap. However, the tank must be vented properly! See threads here, here, and here.

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