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 magic:cool: ).

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 ?). Benefit: 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 (0.00108 x {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 major overflow from your filler cap.

Doesn't seem necessary to have an expansion tank does it? Or have I missed something important?

Any ideas on where I can find that ventectomy thread?

What's the reason for overfilling the tank so the expansion tank is used?

Basil, you have far too much time on your hands, Try building a jigsaw or something similar to take your mind off it

Thanks M8, it's not that I have too much time - honest - it's just that i couldn't get to sleep!!

(BTW my last project was a belt grinder for knife-making and the one before was a AC/DC diode box to convert my cheapo AC welder to DC:) )

On-topic:

Check out: http://inmud.co.uk/page26.htm

http://forums.tdiclub.com/showthread.php?t=140658&highlight=ventectomy

IIRC, it was advised that the vent filling trick wasn't advisable in summer or if the car wasn't going to be driven after being filled up. I never bothered with it on mine as the car was always quicker with less weight than more!

Chris

...I never bothered with it on mine as the car was always quicker with less weight than more!

This is what confused me. Ok, so you make less trips to the petrol station with a larger capacity tank, but if you're doing THAT many miles that trips to the petrol station are a chore, then increasing the capacity by another 10L isn't going to make much of a difference is it?

How much more fuel would the engine consume with the extra weight? How much would the performance be affected?

I have done this expansion valve removal on both my Golf Tdi & SWMBO's Fabia Tdi, it took about 15mins each as follows:

Pictures are from the Fabia, the Golf is exactly the same the only difference is the valve is located to the left of the filler pipe on the Golf.

1. Open fuel filler door.

2. Remove Torx screw.

3. Unscrew fuel cap.

4. Remove door and inner trim by carefully pulling out at screw hole and moving the whole item (door, seal & drain pipe) towards the rear of the car the whole assembly should come out.

5. The plastic drain hose can be pulled out with the assembly.

6. On the right of the fuel filler neck you will see a black plastic bit protruding out about one inch (on the left on a Golf) The valve you wish to remove is inside this bit & white in colour. Using flat screwdriver it's possible to remove the valve by pushing the white tab in and at the same time levering the valve outwards. This is done via the slot where the valve can be seen in the black plastic outer. It should be levered out away from the filler neck while pusing inwards on the white tab. It helps to push on vent "button" from inside filler neck as well.

7. Remove white valve assembly THIS IS IMPORTANT remove the valve, spring & retainer from inside the white valve assembly and REFIT the white assembly minus the valve etc to the car.

8 Refit the filler door & seal, and secure with torx screw.

Enjoy vent free filling !

Filler Door assembly removed.

Filler neck showing white valve halfway removed.

White valve assembly, the black part needs to be removed (not o-rings) before refitting.

Don't bother on mine now/ I prefer to see how far "£ X" gets me, so often just slapin £20 / £30 from empty and see how far it will go. Its looking promising I'll get close to 400 miles from my recent £30 buy.

However do NOT do this mod on a petrol car or you will be looking at a repair bill.

i thought this valve just releasing the air thats trapped in the top of the tank , so does the fuel tank actually have an expansion tank or not ?

I top it off, but then have a m/way drive afterwards.

However with fuel being 95p at one end and 99p at the other end you can see why I do it.

before the ambient temperature rockets up to 35 degrees C so as to avoid the threat of major overflow from your filler cap.

However, is the fuel tank at ambiente temperature? On a sunny day, I'd have thought the reflection of heat from the tarmac onto the underside of the car and probably a few other factors would mean the fuel tank would be above ambient temperature...not sure how much of an effect this would have though.

Rob.

However' date=' is the fuel tank at ambiente temperature? On a sunny day, I'd have thought the reflection of heat from the tarmac onto the underside of the car and probably a few other factors would mean the fuel tank would be above ambient temperature...not sure how much of an effect this would have though.

Rob.[/quote']

Yeah, it's not really a problem, with diesel (NOT PETROL) you get very little expansion at all, certainly nowhere near enough to cause any real world problems.

This is a very common mod with VW owners in the States and to my knowledge no problems have ever been reported & that's with their range of temperatures.

On the plus side in my Golf, (I haven't measured SWMBO's Fabia) I can get an extra 7 litres in the tank now.

That's an extra 70 miles range and "brimming" the tank is no longer the chore it used to be albeit it's a little bit more expensive !

A quick & simple mod for diesel owners.

I use this quite a bit, but only if I drive the car afterwards for some distance to get rid of the extra derv, or if I know it will be parked in the carpark at work where it will be shielded from the sun in particular.

Doing it a little less now with the LPG fitted as that inreased the fuel economy on the derv side by enough to be able to get through a full week without a refill inbetween. This means I can get to my preferred Diesel Extra from Shell quite comfortably.

LPG outlets are still a little thin on the ground in Oxfordshire/Berkshire but I've found enough of them to not have to go totally out of my way

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!

If you dont belive me... go out for a drive and get some VAGCOM logs

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

Which is surely a good arguement for putting/keeping as much fuel in the tank as possible if you want to keep fuel temps low ?

Mmmm, fuel feed FMIC?

Yep but that means more weight which in turn means more load = more heat.

Tricky one

The only thing i regret about ventectomy is that i never did it on my previous TDI, which I drove for 4 years.

On the Fabia I used to be able to add 40 liters without touching the vent **** when the warning peep came on, now its nearly 50 liters, and theres no fiddling with the ****.

That gives me nearly 1000 km on a tankfull, allowing me to postpone filling up until the price is down, which happens once a week here.

When we go on holiday towing a caravan it will give me a whole day of driving to a tankfull, saving the stress of finding a filling station late in the evening.

Must admit it's VERY tempting to do this mod

Must admit it's VERY tempting to do this mod

T.B.H. I'm really very surprised it's not a more popular mod over here.

In the States it's very popular indeed amongst Tdi owners & often the first mod they do. The benefits of having that extra maybe 10 litres or so of fuel in your tank & the extra range that gives you is definitely worth having.

Maybe it'll catch on here in time, who knows, it's certainly useful a very quick & easy mod to do.

What's the point in stuffing your car full of fuel unless it's really cheap or in short supply??

I fail to see the logic in this idea myself.

Fuel stations are a plenty and does it really take that long to fill up?

Fuel does 'go off' so why keep a load of it in your tank, keep it fresh.

The expansion system is fitted for a reason and that's not to fill with extra fuel all the time.

What's the point in stuffing your car full of fuel unless it's really cheap or in short supply??

Fuel does 'go off' so why keep a load of it in your tank' date=' keep it fresh..[/quote']

It never stays in my tank THAT long ... I wish it did.

The expansion system is fitted for a reason and that's not to fill with extra fuel all the time.

The general thoughts on this on the Tdi forum is that it's been fitted to allow for expansion in the petrol models and not been removed in the diesel models.

It's been shown on another thread that the co-efficient of expansion of diesel is unlikely to give as much as an egg cup extra diesel in a full Fabia tank with a temperature rise of 100C ( that's a RISE of 100 C over ambient temp by the way ! ) so the expansion tank really seems to be totally uneccessary on diesel models.

I guess this is also borne out by the many hundereds of people maybe thousands running around with the mod without problem in the States but then each to their own modding is a very individual thing I guess.

Perhaps I just don't use enough then. I don't mind going to the filling station when I need fuel.

Filling up a day early can be a pain

You are looking at a very significant extra usable radius for the car, and some pumps click off FAR too early if you don't vent.

It's definitely not worth it if you do low(er) mileage though.