Filler Cap Button

[miked]

Is that a flap which can be seen in the filler neck which is pushed open by the filler nozzle?
I wonder if the green button is mechanically linked to the flap in some way so you can't get the cap back on if the flap is stuck open?

[citronut]

ANYWAY - What's this little green button for

may be you should pop in the see Barry Aneils at Bourne Citroen, he may be able to shed some light on your green button

[momag46]

I'll have a proper look at it tomorrow and see if I can find out what it does - if I can't, I'll talk to Barry.

Maurice

[qprdude]

Second guess.
It's pushed in by the cap, to open the green flap in the filler neck, and allow the tank to breathe.

[Mandrake]

Seems unlikely. It would never meet evap emissions standards with the fuel tank permanently open to breathe to the atmosphere.

That defeats the whole charcoal canister system where petrol vapours that evaporate inside the tank while parked are stored then burnt when the engine is running.

[qprdude]

Does that apply to diesel? Thought that was only spark ignition engines

[Northern_Mike]

Seems unlikely. It would never meet evap emissions standards with the fuel tank permanently open to breathe to the atmosphere.

Could it work the opposite way - the hole opens when the cap is removed to allow the tank to breathe when being filled? Or even be some sort of one - way valve to allow the tank to such air in as the fuel level goes down but not allow anything to evaporate to the outside?

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[qprdude]

Kind of what I was meaning Mike, allowing the tank to breathe. (I probably didn't put it the right way.
After all. Old petrol caps used to "breathe", otherwise the tank would collapse as the fuel was used. I'm not into all this EVAC stuff, but from an engineering point of view, if you suck liquid out of a tank without air to replace it, there is only one result. You cause a partial vacuum which results in the tank collapsing.

[addo]

Yes, I believe it's to do with the splash and vapour containment. It's not electrical.

[momag46]

Hi all

Barry wasn't at his place this morning (11:00) so I had a fiddle.

It is not linked to the locking system - with the flap open and the filler cap off, I locked the doors and the flap pin popped out to lock the flap (that was obviously open) but with the button pushed in (as when the cap is on) or left out the filler neck flap could be pushed open.
I also did the same exercise starting with the filler cap in place.

Looking at it, it doesn't move very far when the cap is fitted so it doesn't look like it could be mechanical or electrical - could it just be a way of stopping the cap coming undone with vibration?

Maurice

[Stickyfinger]

FYI....1997 Citroen Xantia 1.9 turbo diesel does have a hole for it but no button

[Mandrake]

FYI....1997 Citroen Xantia 1.9 turbo diesel does have a hole for it but no button

Does that car have an alarm ?

[Mandrake]

Kind of what I was meaning Mike, allowing the tank to breathe. (I probably didn't put it the right way.
After all. Old petrol caps used to "breathe", otherwise the tank would collapse as the fuel was used. I'm not into all this EVAC stuff, but from an engineering point of view, if you suck liquid out of a tank without air to replace it, there is only one result. You cause a partial vacuum which results in the tank collapsing.

Petrol caps on modern cars are supposed to be air tight when done up to prevent petrol vapours escaping - that's the point of the evap system.

The breather hole for filling the tank is the square slot visible on the right hand side inside the filler neck - with the cap off that hole is open to allow the tank to fill easily but blocked off when you fit the cap.

Vacuum when consuming petrol from the tank doesn't occur because the charcoal canister purge valve is operated by the ECU periodically while the engine is running - intermittently and briefly at low throttle openings and more regularly at wider throttle. Thus any vacuum generated by fuel being consumed from the tank is relieved into the intake tract.

There is also a pressure relief valve but that only operates in extreme cases, not in normal operation.

[Stickyfinger]

FYI....1997 Citroen Xantia 1.9 turbo diesel does have a hole for it but no button

Does that car have an alarm ?

not a clue, not mine

[wheeler]

Just noticed this thread, xsaras also have this 'button'. They have no electrical connections whatsoever, purley mechanical.
From what I understand when the cap is removed it shuts off a chamber somewhere in the filler neck so that the tank cant be truly filled to 'the brim', when the cap is put back on the chamber is opened again leaving space for expansion of the fuel. if you have ever had a plastic jerry can of petrol on a hot day you will see the pressure that builds up in them. Not sure if its on diesel models aswell but my petrol Xsara has it.