Petrol tank puzzle

[Mike Austin]

Late on Thursday evening, I topped up my petrol tank to the brim, about 3 miles after the warning light came on showing I was on the reserve. According to the pump, 50 litres went in! The most I have ever put in before is 41.4 litres when I was well into the reserve.

I think the tank capacity is only 45 litres. Allowing for the reserve (7 litres), only 38 litres should have gone in. Naturally, I thought the pump was wrong (pump no. 12, South Mimms services on the M25). I told the staff and the next day I contacted BP customer services for a refund.

Today (Sunday) driving back to Bristol, the tank showed full even after about 190 miles! After finishing my journey, the tank still showed 3/4 full after 224 miles. The trip computer registeried 43.7 mpg for the journey (it was slow with many delays - but still not bad for a 2.0i).

So now I am wondering if 50 litres did actually get into the tank. Any explanations?

[Td-eye]

If you brimmed it by venting the nipple at the filler neck either deliberately or by accident you would get approximately an extra 7 litres in your tank. The fuel gauge wouldn't take account of this and would stay at full for longer.

I get this all the time (fuel guage doesn't move until about 150 miles) having done a ventectomy on SWMBO's Fabia !

[Mike Austin]

Originally Posted by Td-eye

If you brimmed it by venting the nipple at the filler neck either deliberately or by accident you would get approximately an extra 7 litres in your tank. The fuel gauge wouldn't take account of this and would stay at full for longer.

I was not aware of a nipple there. I will have a look. But even still, with a near-full reserve, only 38 + 7 = 45 litres should have gone in.

[Lummox]

Either way venting the tank and overfilling is NOT a good idea on a petrol car unless you like your engine management light coming on and an expensive repair bill.

[Mike Austin]

Originally Posted by Lummox

Either way venting the tank and overfilling is NOT a good idea on a petrol car unless you like your engine management light coming on and an expensive repair bill.

If this is indeed what I did, it was completely unwittingly. It doesn't sound a very safe arrangement.

[TeflonTom]

it's a little strange. but mine sometimes does that too... the needle can be at the top but the little symbol on the dash stays on for a few miniutes. but it only does it when filling it right up

[Td-eye]

Originally Posted by Mike Austin

I was not aware of a nipple there. I will have a look. But even still, with a near-full reserve, only 38 + 7 = 45 litres should have gone in.

Yes, 45 litres with a (normally) full tank but then there's an additional 7 litres available by venting (filling the expansion tank) giving a total capacity of 52 litres which for those (like me) still working in old money that's 11.4 gallons instead of 9.9 gallons.

As Lummox says don't try this with a petrol engined vehicle but it's perfectly safe with diesels.

[Tom_vRS]

What warning light would it bring on and why Ross? Just curious like!

[fletch555]

Could somebody post a picture of the nipple button? I'm not sure what it looks like. I pressed some button in before (kinda looked like a pop rivet) but it never came back to it's original position.

[Lummox]

Originally Posted by Tom_vRS

What warning light would it bring on and why Ross? Just curious like!

The engine management light.

The tank breather system is connected to a charcoal canister via a solenoid valve (usually hear this clicking away on petrols). This is so potentially dangerous fuel vapours from the tank can be recycled into the engine, making it safer and more enviromentally friendly. If you over fill the tank the fuel backs up down the vent pipe and fills up the canister making it useless it also ruins the solenoid.

[Tom_vRS]

Cheers for that! Learn something new everyday!

[Td-eye]

Originally Posted by fletch555

Could somebody post a picture of the nipple button? I'm not sure what it looks like. I pressed some button in before (kinda looked like a pop rivet) but it never came back to it's original position.

You can see it, and how to do a "ventectomy" HERE.

[Mike Austin]

Originally Posted by Td-eye

You can see it, and how to do a "ventectomy" HERE.

I have just taken a couple of pikkies of the neck of my fuel taken (an 'unexploded' view). There is a small lever that is up by default, but can be pushed down. I presume it is pushed down when the pump nozzle is inserted.

What could I have done wrong? I have driven Fabias for 5 years. I estimate this equates to about 200 refuels. I have never been able to put in 50 litres. And bearing in mind that I already had nearly a full reserve, the 7 litre vent capacity does not explain this.

[Td-eye]

Originally Posted by Mike Austin

I have just taken a couple of pikkies of the neck of my fuel taken (an 'unexploded' view). There is a small lever that is up by default, but can be pushed down. I presume it is pushed down when the pump nozzle is inserted.

Yup, that's indeed the vent nipple.

Originally Posted by Mike Austin

What could I have done wrong? I have driven Fabias for 5 years. I estimate this equates to about 200 refuels. I have never been able to put in 50 litres. And bearing in mind that I already had nearly a full reserve, the 7 litre vent capacity does not explain this.

Unless your reserve light is coming on late ? ..... ie with only a couple of litres left that, or the garage's diesel pump being wrong (unlikely as they are subject to strict checks by weights and measures) is about all I can come up with. Certainly, when I brim the Fabia tank now it takes about 150 miles before the gauge moves off the full mark.

PS. Nice choice of colour of car !

[Mike Austin]

Originally Posted by Td-eye

Yup, that's indeed the vent nipple.

And it should go down when the nozzle is inserted?

Originally Posted by Td-eye

Unless your reserve light is coming on late ? ..... ie with only a couple of litres left that, or the garage's diesel pump being wrong

This is what I reckon (althought it would have been petrol, not diesel) - whether by accident or otherwise. It took a long time to fill up, so I reckon there may be some return valve on the pump itself and it was going back to the pump.

Originally Posted by Td-eye

PS. Nice choice of colour of car !

Yes. Both my Fabias have been this colour. Skoda removed it from the last brochure for the Mk1s - and it is not available for the Mk2 either.

[Mike Austin]

Originally Posted by Mike Austin

And it should go down when the nozzle is inserted?

I was just reading through the full thread here and it seems that this 'nipple' should not go down when the nozzle is in but only when the filler cap is replaced. It seems to me that it may be possible to push this down accidentally with the nozzle. But if this happens, would it not be easier for the fuel to spill out of the neck of the tank rather than flow through the narrow hole in this nipple? (I presume it does flow through it rather that round it.)

Even for diesel drivers who have removed the complete nipple assembly, there seems more of a gap between the nozzle and the neck than where this assembly was. How do they ensure that the fuel goes into the vent tank rather than splash out through the neck of the tank?

[Lummox]

The 'nipple' doesnt let fuel in, it lets air out so the tank can be filled further.

[Mike Austin]

Originally Posted by Lummox

The 'nipple' doesnt let fuel in, it lets air out so the tank can be filled further.

A complete misunderstanding on my part. I thought that there was a further 7 litres capacity in a vent tank and that this was accessible when this nipple assembly was removed or the nipple was pushed in. Could you explain what goes on here?

[Lummox]

There is, but petrol wont enter it unless the air that is in the vent tank is allowed to escape.

[Mike Austin]

Originally Posted by Lummox

There is, but petrol wont enter it unless the air that is in the vent tank is allowed to escape.

OK. Let me get this straight. Because the 'nipple' seems to have a hole in the end of it, and because it is called a 'nipple', I assumed fuel or vapour passed through it. But I was wrong.

I presume there is a vent volume in the tank something like a diving bell. When the tank fills up normally, air is trapped in this 'bell'. The 'nipple' is just a lever that opens a venting valve at the top of the vent volume. OK so far?

I presume the fuel cap pushes this nipple down? So when this is done, the vent volume is vented and fills up? If I were to remove the cap again, could I now put more fuel in?

It seems the nipple can be accidentally pushed down by the nozzle from the petrol pump. This means the vent volume is constantly vented and fills up during 'normal' refuelling. Correct?

Could this cause problems and/or damage? If so, how?

[Lummox]

Yes it can cause damage as I pointed out a few posts up.

[Mike Austin]

Originally Posted by Mike Austin

Could this cause problems and/or damage? If so, how?

Originally Posted by Lummox

Yes it can cause damage as I pointed out a few posts up.

Like here?

Originally Posted by Lummox

Either way venting the tank and overfilling is NOT a good idea on a petrol car unless you like your engine management light coming on and an expensive repair bill.

Your wonderful economy with words is let down by a low mileage.

Can anyone else tell me what damage could have been caused by the accidental triggering of the vent nipple when refuelling?

[Td-eye]

Originally Posted by Mike Austin

OK. Let me get this straight. Because the 'nipple' seems to have a hole in the end of it, and because it is called a 'nipple', I assumed fuel or vapour passed through it. But I was wrong.

I presume there is a vent volume in the tank something like a diving bell. When the tank fills up normally, air is trapped in this 'bell'. The 'nipple' is just a lever that opens a venting valve at the top of the vent volume. OK so far?

Correct !

Originally Posted by Mike Austin

I presume the fuel cap pushes this nipple down? So when this is done, the vent volume is vented and fills up? If I were to remove the cap again, could I now put more fuel in?

No, because the valve the nipple is attached to is spring loaded and will close when the cap is removed again. Removing the valve completely (ventectomy) allows the air to escape out the expansion tank as you fill up & allowing the expansion tank to fill with extra fuel.

Originally Posted by Mike Austin

It seems the nipple can be accidentally pushed down by the nozzle from the petrol pump. This means the vent volume is constantly vented and fills up during 'normal' refuelling. Correct?

Correct !

Originally Posted by Mike Austin

Could this cause problems and/or damage? If so, how?

Not in diesel engined cars, in petrol engined cars yes, because petrol expands much, much more than diesel and needs the expansion tank & it can cause damage for the reasons given by Lummox (who may seem to you to be economical with words, but he certainly knows his subject believe me!)

viz.....

Originally Posted by Lummox

The tank breather system is connected to a charcoal canister via a solenoid valve (usually hear this clicking away on petrols). This is so potentially dangerous fuel vapours from the tank can be recycled into the engine, making it safer and more enviromentally friendly. If you over fill the tank the fuel backs up down the vent pipe and fills up the canister making it useless it also ruins the solenoid.

This only applies to petrol engined cars.

HTH

[Lummox]

Originally Posted by Mike Austin

Like here?

Your wonderful economy with words is let down by a low mileage.

Spotted above, need to read a little more

[Mike Austin]

Originally Posted by Mike Austin

I presume the fuel cap pushes this nipple down? So when this is done, the vent volume is vented and fills up? If I were to remove the cap again, could I now put more fuel in?

Originally Posted by Td-eye

No, because the valve the nipple is attached to is spring loaded and will close when the cap is removed again. Removing the valve completely (ventectomy) allows the air to escape out the expansion tank as you fill up & allowing the expansion tank to fill with extra fuel.

My reasoning is like this. When I fill up normally, there should be pressure in this vent volume keeping fuel out. When I replace the filler cap, this volume is vented, so it (partly) fills with fuel. This fuel comes from the main volume of the tank. There is room to put more in - maybe not the full 7 litres, but some of it.