Many people apparently depress the little nipple/tab thingie in order to get more diesel into the fuel tank. It diverts fuel into an "expansion tank". It's apparently something of an art form at the pumps.....(Xav is well-known to be a dab hand at this black art).

There's some discussion about if this is safe to do in hot weather (based on common sense: everything expands more then) and contradictingly if diesels really need such an expansion tank at all.

Apparently there was a thread on "ventectomy" (not working now) that showed you how to botch up/ modify your tank filler neck to easily take another several litres (correct me someone is it 8-11l ?). More range, less trips to the pumps.

I looked it up- the coefficient of expansion (per degree C) for diesel fuel is 0.00108, meaning that for every degree rise in temperature you get 0.00108% more diesel volume.

So if you filled your tank using the filler nipple trick to the max ++ say 56l at 20 degrees ambient and the temperature suddenly shot up to 35 degrees ambient, this would translate into (0.00108x {35-20} x 56) litres more fuel in the tank. That works out to 0.9072l more fuel volume.

Depending on how you drive (vRS combined "mpg" is 5.4l/100km), you would have to get away some (100/5.4 x 0.9072) = 16.8km before the ambient temperature rockets up to 35 degrees C so as to avoid the threat of overflow from your filler cap.

Doesn't seem necessary does it?

You worry too much about things!!

(Einstein's theory etc snipped)

Doesn't seem necessary does it?

No !

Basil... your missing one big point... (just so i can scare everyone)

With the diesel engine, its constantly returning un-used fuel to the tank... which gets HOT.... eg 70-100C hot.

so what does your caclulation say about the temp rising 90c?

thats why a car can be noticeably more lazy when the tank is almost empty.

After intake temps, fuel temps are the next big issue with diesel engines as theres only a tiny little heatsink on the bottom of the car to cool the returnline... its useless!

I must stop reading this thread as Vasectomy summer safety

I must stop reading this thread as Vasectomy summer safety

OMG up until now I actually thought it read that

I'm glad I'm not the only one, thanks for that Walks .......I owe you

Basil... your missing one big point... (just so i can scare everyone)

With the diesel engine' date=' its constantly returning un-used fuel to the tank... which gets HOT.... eg 70-100C hot.

so what does your caclulation say about the temp rising 90c?

thats why a car can be noticeably more lazy when the tank is almost empty.

After intake temps, fuel temps are the next big issue with diesel engines as theres only a tiny little heatsink on the bottom of the car to cool the returnline... its useless![/quote']

Fair point, but in practice in relation to the mod itself it doesn't seem to cause any problems unless of course, you can point me to someone who has done this mod & is experiencing problems.

No doubt if you brim your tank with cold (ish) diesel from the pumps it will take a while and a fair bit of fuel having been burnt off before the mass of new fuel in your tank reaches the temperatures you are talking about (if ever, with a near full tank).

In fact, in my view (and those of many others on the Tdi forum) expansion tanks are totally uneccessary in a diesel given that diesel will only expand (according to "Basil") by the coefficient of expansion of diesel which is (per degree C) only 0.00108, meaning that for every degree rise in temperature you get 0.00108% more diesel volume."

So for a 100 degree rise in temperature you would see only a 0.1% increase in the volume of diesel which would equate to not even an egg cup full extra in a Fabia tank.

To achieve a 100 degree rise in temperature in that mass of diesel would take the energy of several litres of diesel being burnt by which time obviously there would be space in the tank for expansion which is why the expansion tanks are totally uneccessary in diesel engined cars & better used to carry extra fuel.

Now in a petrol engined vehicle that's different .... !!!

In practice filling the expansion tank which actually gives a greater mass of cold fuel in the tank and a greater range causes no known problems.