Today I filled one tank completely. I drove back on the other tank. After 2 hrs in the garage, I returned to see petrol on the floor, underneath the drain tube, plus the whole garage reeked of gas.
Should the lids be ventilated?
I don´t think so, as the lids are the origina, star like lids.
What is going on…?
Expansion or there’s a leak 
The tanks have vent tubes built into them. Item 7 on plate 75.
They’re pretty small ID, so can get blocked fairly easily.
Thanks, as always, Andrew.
So your thought is that with a blocked vent pipe, petrol would be pushed out …where?
The petrol mark on the floor is just below the vent tube aperture. But thatb pipe is the spill line from the area around the filling neck and lid.
I will check again this morning.
But I could understand this happening on a hot summer day, but today is a cloudy, 9 deg day.
All my fault, and you are right. I had filled a bit too much, and some took the ventpipe exit. After airing the garage, there is no ,ore smell.
I had the Mk10 arrangement in mind, where there is a hole near the filler neck. Then a rubber tube,I think, and then a fixed metal pipe, easily seen from below. I put compressed air in these once a year, and check that water exits as it should.
Peder
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Gasoline is rather cool when stored underground at the station and expands somewhat when pumped up and put in the vehicle tank. Depends on the ambient temperature the car is parked in but if the tank is very full it could overflow. I usually run a few miles on the just filled tank to bring the level down a bit.
In the fill tube there will be a small volume of fuel vapor above the liquid fuel, and this vapor expands very much as it warms up, and if the star-like cap is a perfect seal, the vapor will push liquid fuel out the vent tube if it is located below the liquid level when the tank is full.
This is why the vent is located high up and run to a carbon canister on emissions models like the XJ.
The very same thing once happened to me with my Mark V, early morning fillup and sitting in the hot sun all morning, made a big fuel stain on the parking lot.
If the compressed air does not clear that vent tube, try using an old flexible speedometer cable to pump up and down into the vent line. You might be amazed what gets jammed in there after 50 years. Also check under the inspection portal in the trunk to see if your seals are weeping and also if your pump lines are slightly cracked.
I use one way Purloator/Facet pumps so that fuel does not get pushed from one tank into another which might be happening?
Gerard