Many thanks to td-eye on Briskoda for this guide which helped me create this one!!! All I did was take a few extra pics and add a little wording ;)
First of all, only attempt this on a DIESEL. If you have a petrol you run the risk of causing very expensive damage to your car.
What this mod does, is allows the expansion tank to be filled under normal refuelling. The expansion tank is there to allow for the expansion of diesel when ambient temps rise, but many people have done this (including a Briskoda member in the Arab Emirates) and have never experienced diesel spillage due to heat. Its worth remembering that you will always drive the car after refueling, so there will always be some excess capacity.
The expansion tank is roughly 5 litres or so, so good for circa 50 miles extra per refill.
1) Open your filler flap. You will see a T20 torx screw that needs to be removed.
2) Then, pull the whole filler neck with flap housing towards the rear of the car and outwards, and it will come away easily. There is a hose attached to the bottom of this that is just an overspill hose, and this is removed complete with the housing.
3) On the left of the filler you will see a black pipe, and also the "nipple" inside the filler neck.
4) On the underside of the black pipe there is a small slot with a tab that needs to be pushed with a screwdriver blade. When pressed in (its very easy), push on the nipple inside the filler neck and out will slide the valve assembly.
5) You need to remove the nipple/valve, spring and retainer from the valve assembly. You need to prise the end of the housing over the tabs, this really is very easy, and the plastic should be nice and supple.
6) Simply reinsert the valve assembly (without the spring, nipple/valve and retainer) back where you took it out of, and feel it lock into place.
7) Its worthwhile cleaning the hose, filler neck etc while its all accessible, there will be lots of road grime on it all. Make especially sure you don't drop any poo into the tank.
8) Slide the filler flap housing back onto the car, screw the T20 torx back into place, and admire your handywork.
You will now get a satisfying whoosh as the tank gets nearly full. Take care, but in my experience the fuel pump cut off will still work properly, only now you will have an extra 5l of fuel in your tank.
