Hmmm, but my interpretation is that the tank vents into the filler neck, so the system balances in that way.

Of course you are right - if diesel is used by the engine, then where is the air coming from to replace that volume of fuel in the tank???

The expansion tank vents back into the filler neck on my Fabia & Golf and is done via a one way valve which is there to prevent fuel getting back down the vent pipe into the expansion tank should the tank be brimmed. That valve can however, be released by moving the nipple at the neck of the filler, allowing the expansion tank to be filled by releasing the air trapped in there or, by removing the valve completely (a "ventectomy" ) which allows the air to escape from the expansion tank automatically when filling.

The Fabia in common with the Golf & presumably many other V.A.G. vehicles also has a vented filler cap which keeps the fuel tank closed relative to the atmosphere, but permits ambient air to enter the fuel tank to compensate for the low pressure which may be caused in the fuel tank due to fuel being sucked in by the fuel pump, i.e. it's a "one way" valve allowing air to be drawn in to the tank through the cap.

It is for this reason ie. the air cannot get out via the cap (neither can fuel) that an expansion tank is fitted, this normally leaves an air space (even in a brimmed fuel tank ) which will compress slightly should the fuel in the vehicles tank expand.

Obviously, as soon as you use any fuel from your tank even more air space is created due to air being drawn in through the fuel cap to fill the space previously occupied by fuel.

In diesel engined vehicles the expansion tank can be used as extra storage space for diesel because diesel fuel's co-efficient of expansion is so low that there is virtually no meaningful diesel expansion going to take place in your tank in every day use on this planet.

However, it's a totally different story with petrol which will expand significantly in our climates and so for this reason and also for the reason given by Lummox earlier, this mod should NOT be done on petrol engined vehicles.

This sort of phenomenon will be known to anyone who has ever opened a Jerry can on a hot day that has been brimmed with petrol, there will likely be a significant release of pressure. Doesn't happen with Jerry cans filled with diesel though !

H.T.H.

( you can't fill up two things in the same transaction at a petrol station

A mate and myself filled two motorbikes up side by side last weekend... and I've filled up a few fuels cans at the same time as filling my car...

The expansion tank vents back into the filler neck on my Fabia & Golf and is done via a one way valve which is there to prevent fuel getting back down the vent pipe into the expansion tank should the tank be brimmed. That valve can however, be released by moving the nipple at the neck of the filler, allowing the expansion tank to be filled by releasing the air trapped in there or, by removing the valve completely (a "ventectomy" ) which allows the air to escape from the expansion tank automatically when filling.

The Fabia in common with the Golf & presumably many other V.A.G. vehicles also has a vented filler cap which keeps the fuel tank closed relative to the atmosphere, but permits ambient air to enter the fuel tank to compensate for the low pressure which may be caused in the fuel tank due to fuel being sucked in by the fuel pump, i.e. it's a "one way" valve allowing air to be drawn in to the tank through the cap.

It is for this reason ie. the air cannot get out via the cap (neither can fuel) that an expansion tank is fitted, this normally leaves an air space (even in a brimmed fuel tank ) which will compress slightly should the fuel in the vehicles tank expand.

Obviously, as soon as you use any fuel from your tank even more air space is created due to air being drawn in through the fuel cap to fill the space previously occupied by fuel.

In diesel engined vehicles the expansion tank can be used as extra storage space for diesel because diesel fuel's co-efficient of expansion is so low that there is virtually no meaningful diesel expansion going to take place in your tank in every day use on this planet.

However, it's a totally different story with petrol which will expand significantly in our climates and so for this reason and also for the reason given by Lummox earlier, this mod should NOT be done on petrol engined vehicles.

This sort of phenomenon will be known to anyone who has ever opened a Jerry can on a hot day that has been brimmed with petrol, there will likely be a significant release of pressure. Doesn't happen with Jerry cans filled with diesel though !

H.T.H.

Couldnt, and didnt say it better myself

The expansion tank vents back into the filler neck on my Fabia & Golf and is done via a one way valve which is there to prevent fuel getting back down the vent pipe into the expansion tank should the tank be brimmed. That valve can however, be released by moving the nipple at the neck of the filler, allowing the expansion tank to be filled by releasing the air trapped in there or, by removing the valve completely (a "ventectomy" ) which allows the air to escape from the expansion tank automatically when filling.

The Fabia in common with the Golf & presumably many other V.A.G. vehicles also has a vented filler cap which keeps the fuel tank closed relative to the atmosphere, but permits ambient air to enter the fuel tank to compensate for the low pressure which may be caused in the fuel tank due to fuel being sucked in by the fuel pump, i.e. it's a "one way" valve allowing air to be drawn in to the tank through the cap.

It is for this reason ie. the air cannot get out via the cap (neither can fuel) that an expansion tank is fitted, this normally leaves an air space (even in a brimmed fuel tank ) which will compress slightly should the fuel in the vehicles tank expand.

Obviously, as soon as you use any fuel from your tank even more air space is created due to air being drawn in through the fuel cap to fill the space previously occupied by fuel.

In diesel engined vehicles the expansion tank can be used as extra storage space for diesel because diesel fuel's co-efficient of expansion is so low that there is virtually no meaningful diesel expansion going to take place in your tank in every day use on this planet.

However, it's a totally different story with petrol which will expand significantly in our climates and so for this reason and also for the reason given by Lummox earlier, this mod should NOT be done on petrol engined vehicles.

This sort of phenomenon will be known to anyone who has ever opened a Jerry can on a hot day that has been brimmed with petrol, there will likely be a significant release of pressure. Doesn't happen with Jerry cans filled with diesel though !

H.T.H.

Well said that man. :thumbup:

Cheers

Dave.

(a) they dont make diesel V-power (as far as i'm aware)

( you can't fill up two things in the same transaction at a petrol station

They sell V-Power at my local Shell garage and I've been using it for about 6 months now

Why would it not be insurance friendly???

Because you are modifying your car AND it is not a 'proven' mod along the lines of adding better wheels/tyres etc.

If you tell your insurance company, 'Oh, and I've just played around with the fuel tank so I can store more in it' do you not think they will (in their very conservative way) think, 'Oh bugger, what the hell does THAT mean?'

Adding leather or bigger wheels or an air intake is one thing, but tinkering with the fuel supply DOES seem at first glance to offer a lot of safety implications.

I'm not saying don't do it; I'm just saying that in their very conservative risk approach, insurance companies are going to be very nervous (through ignorance) if you tell them that you have tinkered with the fuel intake on your car (even if you explain to them EXACTLY what you have done).

Just a thought...

A mate and myself filled two motorbikes up side by side last weekend... and I've filled up a few fuels cans at the same time as filling my car...

You do, of course, fill up your car AND the 5 litre can AT THE SAME TIME, ie one transaction.

Because you are modifying your car AND it is not a 'proven' mod along the lines of adding better wheels/tyres etc.

If you tell your insurance company, 'Oh, and I've just played around with the fuel tank so I can store more in it' do you not think they will (in their very conservative way) think, 'Oh bugger, what the hell does THAT mean?'

Adding leather or bigger wheels or an air intake is one thing, but tinkering with the fuel supply DOES seem at first glance to offer a lot of safety implications.

I'm not saying don't do it; I'm just saying that in their very conservative risk approach, insurance companies are going to be very nervous (through ignorance) if you tell them that you have tinkered with the fuel intake on your car (even if you explain to them EXACTLY what you have done).

Just a thought...

Yes but you don't have to do the mod to actually fill the expansion tank though..... I've not done the mod as to be honest i can't be bothered to take the filler inlet to bits etc.... i just fill the expansion tank by holding the valve/switch in while filling the car.

Where does this leave any insurance queries??? I wouldn't expect to tell my insurance that i fill to the brim everytime i fill up. And in essence i'm doing exactly the same as what happens when the mod is carried out..... Lets face it all this mod does is stop you having to hold the switch while filling up. I'll still just fill to the brim while holding the switch everytime.

Cheers

Dave.

You do, of course, fill up your car AND the 5 litre can AT THE SAME TIME, ie one transaction.

I've seen pairs of bikers fill up together one fills up then replaces the hose then the other picks it up and fills up then they go in and pay.

In fact in busy periods i've filled up then moved the car forward and the person behind me has been filling up before i've even paid.

I was under the impression all garages worked this way nowadays.

Cheers

Dave.

In fact in busy periods i've filled up then moved the car forward and the person behind me has been filling up before i've even paid.

They wont do that around this way anymore, Tescos dont allow it thats for sure. Not sure if its due to the video system, pay at pump or what but there are notices saying dont move the car until you have paid.

I've not done the mod as to be honest i can't be bothered to take the filler inlet to bits etc.... i just fill the expansion tank by holding the valve/switch in while filling the car.

Can you tell me exactly how to do this please?

If you just want to fll the expansion tank without doing the mod just hold the nipple on the filler neck down while filling your tank.

Can you tell me exactly how to do this please?

When you have the filler cap off if you look to the right of the opening you'll see a little spring loaded switch. Just hold it forward with the end of the diesel pump gun/nozzle and you'll fill the expansion tank.

I usually fill as normal then when it gets to the top i hold the switch in and you hear the air escaping and the diesel disappear back down the inlet. Then just continue filling whilst holding the switch in till you fill the expansion tank.

Cheers

Dave.

Thanks - will give it a go next fill up

If you hold the valve while filling then that's not a permanent mod, so you should be okay.

Just don't crash as you leave the garage forecourt with a brimmed tank!

When you have the filler cap off if you look to the right of the opening you'll see a little spring loaded switch. Just hold it forward with the end of the diesel pump gun/nozzle and you'll fill the expansion tank.

I have had a look with the filler cap off with a torch and I cannot see anything other than a hole to put the pump nozzle into :confused:

Any chance of a photo showing the location of the switch??

I don't think that the new Octavia VRS has this. :(

What is being described is the black part of the valve shown in this picture which protrudes into the filler neck on earlier model Skodas.

I may be wrong but I don't think this valve was fitted to the later model Skodas. :confused:

I have had a look with the filler cap off with a torch and I cannot see anything other than a hole to put the pump nozzle into :confused:

Any chance of a photo showing the location of the switch??

Here ya go....... It's the bit on the right hand side inside the opening.... You just keep it pushed away from you while filling up..... As mentioned above it may be that the Octy II doesn't have it.

Cheers

Dave.

It is definitely not there on my car :(

Oh well, standard filling only for me then

Mk 2 Octavia's do not have the removable 'switch'.

Bugger! Just read four pages only to discover it doesn't apply to me.

Lets face it all this mod does is stop you having to hold the switch while filling up. I'll still just fill to the brim while holding the switch everytime.

Which switch is this?