Venting the fuel tank

[kaanage]

Standard apology if the subject has been covered before and I haven't been able to find the relevant thread(s)

Does anyone know if the MkVI Golf can have the fuel tank vented to allow additional fuel to be pumped in after it reaches the normal full state?

I know that the MkIV can do this and my MkVI has a very noticable sloshing sound immediately after I fill it up, so I know there is vent space still in the tank.

[Lams]

why do you want to do this... the vent space is there for a reason!!

[Corey_R]

Some people like to pump in more than that max and fill the vent space if they're going to be driving long distances immediately after filling up. So for Golf tank is 55L (60 on the R), but you could possibly fit up to another 5L in there if you can vent it.

But yeah...

[kaanage]

Diesel doesn't expand and evaporate like petrol

[craig_the_german]

I've not tried this on my MK6 but on my MK4, if you fill the tank past the first shut off on the bowser nozzle, the fuel gauge gets "stuck" on full and you need to reset it by disconnecting the battery.

[craig_the_german]

This is a post from the FAQ area on the MK4 forum.......maybe filling it right to the top is not such a good idea.

Oneofthegreats
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On Hiatis Join Date:Oct 2005
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How long has it been doing it for?

It may be just an electrical glitch in the instrument cluster. Earlier BMW's had this problem when the owner would fill them right up to the petrol cap.

This was the fix. Disconnect both the postive & negative battery lead's & bridge the two lead's with a screwdriver or something metallic for a roughly 10 second's or touch the two together if they will reach. Don't bridge out the battery, just the leads.

This decharges/de-energizises any capacitor's in the vehicles electrical system. It may sound strange but this has fixed many a wierd electrical problem.

[wai]

All fuel tanks have to be vented, and not directly to atmosphere. Petrol vehicles generally have a carbon cannister, but they can be vented back to the crank case where the fuel vapours are purged through the positive crankcase ventilation. Diesel vehicles will also have some similar system. There is an ADR covering fuel system venting.

Venting is not just to cope with expansion, but to allow air in as the fuel level falls. On a fully sealed system, the tank would eventually collapse. If you are going to be driving a fair distance immediately, then it is OK to try and add more after the first shut off. If you are just going to try and get a few litres more, then you can have issues with excess fuel spilling. You should remember that the fuel can be quite a few degrees cooler in the underground bulk fuel tanks. Not only are the tanks covered in soil that insulates them, underground tanks are also double skinned with the space under slight vacuum. This is used as an indicator that the main tank has failed if the vacuum in the space disappears. On a hot summers day, you can get a significant expansion of the fuel.

[kaanage]

I've not tried this on my MK6 but on my MK4, if you fill the tank past the first shut off on the bowser nozzle, the fuel gauge gets "stuck" on full and you need to reset it by disconnecting the battery.

Doesn't happen IME with my Polo (same venting arrangement as the MkIV Golf).


All fuel tanks have to be vented, and not directly to atmosphere. Petrol vehicles generally have a carbon cannister, but they can be vented back to the crank case where the fuel vapours are purged through the positive crankcase ventilation. Diesel vehicles will also have some similar system. There is an ADR covering fuel system venting.
No arguement


Venting is not just to cope with expansion, but to allow air in as the fuel level falls. On a fully sealed system, the tank would eventually collapse. .
No arguement


If you are going to be driving a fair distance immediately, then it is OK to try and add more after the first shut off. If you are just going to try and get a few litres more, then you can have issues with excess fuel spilling. You should remember that the fuel can be quite a few degrees cooler in the underground bulk fuel tanks. Not only are the tanks covered in soil that insulates them, underground tanks are also double skinned with the space under slight vacuum. This is used as an indicator that the main tank has failed if the vacuum in the space disappears. On a hot summers day, you can get a significant expansion of the fuel.
Not applicable for diesel.

[Lams]

coef of expansion for diesel is around 9.5x10-4/K
if your temp difference is 20C between the tank in the ground and in the car then that's roughly 1L for 50L

but of course you'd be driving away from the servo. But say you fill it up, and park it in the sun, this vent space is there to cater for this expansion.

The machinery we design at work we usually allow 10% of space in the tank for air for this reason

[kaanage]

Yes, the expansion rate is 0.095% / degrees Celcius, in more common terminology, or about half that of petrol plus the vapour pressure is negligible vs petrol between 0 - 70 degrees Celcius hence the minimal requirement for vent space (unless someone was to absolutely fill it to the brim and then immediately park the car for several hours on a 40+ degree day).

Now that we have got that discussion out of the way, can anyone answer my original question?

Does anyone know if the MkVI Golf can have the fuel tank vented to allow additional fuel to be pumped in after it reaches the normal full state?
A simple "no that is not possible" would be sufficient if that is the case.

[wai]

Not applicable for diesel.
I'll have to remember that for the next 100 underground bulk diesel fuel tank installations I design.

[kaanage]

Thanks for you "contribution" - we are talking about totally different applications :P
Can you answer my question one way or the other.:confused:

[wai]

I thought I did. All tanks have to be vented otherwise they will collapse when you pump the fuel out of it. If you fill the tank to try and get more fuel into it, then if it expands beyond the available vent space, the excess fuel will end up wherever the vent system goes to. This varies from manufacturer to manufacturer.

On page 215 of my Caddy manual, it says under Refuelling, "The fuel tank is full when the properly operated automatic filler nozzle clicks off for the first time." It goes on to say "Do not continue filling up after it switches off! The expansion space in the tank will otherwise fill up and the fuel could spill out. This could also happen when the fuel warms up and expands."

It cannot be any clearer than that.

[Idle]

I thought I did. All tanks have to be vented otherwise they will collapse when you pump the fuel out of it. If you fill the tank to try and get more fuel into it, then if it expands beyond the available vent space, the excess fuel will end up wherever the vent system goes to. This varies from manufacturer to manufacturer.

On page 215 of my Caddy manual, it says under Refuelling, "The fuel tank is full when the properly operated automatic filler nozzle clicks off for the first time." It goes on to say "Do not continue filling up after it switches off! The expansion space in the tank will otherwise fill up and the fuel could spill out. This could also happen when the fuel warms up and expands."

It cannot be any clearer than that.

In days gone by (before they started putting expansion space in tanks) I've often seen fuel running down the gutter from a just-filled tank.

All it would have taken was a cigarette butt...

Having said that, I've also been known to run my rear wheel up on a brick to cram a bit more fuel in before heading off on a long run — relying on consumption to outstrip expansion — which fortunately it always did.

When filling the TDI I'll usually trickle diesel in until the second click, which gets rid of most of the foam (or at least moves it into the expansion space, where of course it does no harm and eventually condenses) and hasn't caused any problems so far.

[kaanage]

All tanks have to be vented otherwise they will collapse when you pump the fuel out of it.
Derr.. Or the pump simply won't pump fuel against the lowered pressure.

I've found from another site that the MkVI Golf's fuel tank cannot be vented when filling. Thanks for everyone's "help"

[Idle]

Derr.. Or the pump simply won't pump fuel against the lowered pressure.

I've found from another site that the MkVI Golf's fuel tank cannot be vented when filling. Thanks for everyone's "help"

I don't quite get that — if it can't be vented whilst filling, where would you suppose the air goes?

It doesn't start to be compressed until the fluid level reaches the expansion (vent) space, so the displaced air simply comes back up the filler tube until such time as inflow ceases and the filler cap is replaced.

It's my understanding that this is how the auto shutoff nozzles work — when fluid reaches the vent hole in the end of the nozzle the back pressure triggers the release. That's also why many poorly designed filler tubes won't take the full flow and have to be filled slowly, and why you can usually get a few more litres in with a trickle flow.