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Fuel Leak

I have just filled up my S3 diesel before picking the children up from school. When I returned, I noticed a small pool of diesel on the road roughly below the fuel tank. On closer inspection, there's a small drip on one of the three bolt heads that presumably keep the tank in place.

What's likely to be the most likely cause of the leak, and do I need to worry about it?

If it only leaks when you have just filled it up, it is likely to be one of the seals at the top where the sender unit etc is. If it happens with the tank more than half full, it could well be the seam around the middle. Either way, it's a drop the tank out job to sort it out.

I *think* a leaking fuel tank is an MOT fail.
If they are anything like the 110 tanks, they leak adventually, mines a 86 and the rear tank leaks like a very leaky thing :(

The first thing to do is open up the inspection panel on the tub floor and see if you can see diesel on the top of the tank. You may be lucky and spot the leak immediately, but if it's not obvious, wash off the top of the tank with some washing up liquid and a hose and monitor for leaks.

Check all the pipe unions and clips are tight and that none of the hoses are perished. In all likelihood, Bernie's suggestion of the sender unit gasket leaking when the tank's full will be right. The sender's flat plate sits on a lip on the tank with a flat rubber gasket between. A retaining ring holds the parts together by rotating under three bent tabs on the tank collar, a bit like a rad cap. If these tabs have bent away a little or if there is any dirt on the gasket, it'll leak. Use a new gasket and lightly smear it in grease before fitting, but DO NOT use any other sealant on it - the diesel finds its way through sealants and they then work against the rubber gasket.
I had the same thing with my 109SW - first time I've filled it to the top since buying it 3mths ago, parked up on the driveway only to find a puddle when I returned later. Turns out the exhaust has been vibrating against the tank and bashed a hole in the side about a third of the way from the top. Nuts! Ciphoned 20litres into a Jerry and the leak seems to have abaited.
I always noticed a petrol smell after refilling my petrol 88". Then there came a drip. It was a relatively new tank and I traced it to a small split in the filler hose which was starting to perish and became porus. To renew was an exercise and a half as you need strong hans (three off) and there is a real design flaw in where the hose runs as it can rub on a shar edge. 3 hours later all fixed but do also check as mentioned the sender unit etc as mine was also leaking from those gaskets.

The problem was only there when above 3/4 full so main seam should be OK.

Yep check the sender and the filler pipe into the tank. If the tank itself is leaking I would replace it rather than repai. New ones aren't expensive and the hardest part about fittng it is, as Blott says, the filler pipe! Last time I did one of them my hands were cut to ribbons and ached for days afterwards!
If you do come to change the tank be prepared for a nasty surprise on the side of the chassis behind where the tank is! Amazing really......a simple fuel leak has now turned into a ground up rebuild! I love series trucks!:D
Agree totally with everything in the last post. There is a definite "trap" between the tank and chassis rail. Caught mine in time whilst it was still surface and not deep rooted corrosion.

A week after changing the tank filler pipe my hands and wrists still look as if next doors moggie had a go at me !

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