Filler Cap Button

[momag46]

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.

Thanks for that - this seems like the probable answer.

I spoke to Barry today and he thought it was to allow fumes to be released BUT the above looks more likely.

When I originally asked the question I expected an instant answer but I was wrong.
Just when you think 'everything you want to know about Citroens is on this site' you're let down

Maurice

[Hell Razor5543]

When I originally asked the question I expected an instant answer but I was wrong.
Just when you think 'everything you want to know about Citroëns is on this site' you're let down

Maurice

That's not fair! Every question has to have a first time of asking, and then everybody goes off to find the answer. At least you got several to choose from.

[Mandrake]

I agree wheelers explanation seems most plausible, but I think it's by no means certain. If so why do some Xantia's not have this button ?

[Hell Razor5543]

If the difference is between diesel and petrol tanks, then probably because petrol is more volatile, and so has a faster evaporation rate than diesel, so needs a bigger vent than a diesel tank.

[wheeler]

On modern petrol cars fuel vapour is 'vented' into a charcoal cannister, the ecu can then recylce it back into the engine when it sees fit via a small electrovalve.

[Mandrake]

On modern petrol cars fuel vapour is 'vented' into a charcoal cannister, the ecu can then recylce it back into the engine when it sees fit via a small electrovalve.

Yep, and a Xantia is modern enough to have this.

The canister and electrovalve is under the left hand front wing right up near the top, at least on the V6, but I suspect its in the same place on all petrol Xantia's as there isn't really anywhere else to fit it as the canister is quite large!

A pipe comes through a hole in the wing past the battery to connect it to the inlet manifold at the throttle plate. Occasionally at idle you'll hear a "click click click click" coming from the front left wing - that's the electrovalve at work releasing stored vapours to be burnt.

[Hell Razor5543]

I wonder. Is it possible that the green button locks the flap in the closed position, thereby sealing the tank and preventing fumes getting out through the filler point, and limiting petrol escaping in the event of an accident?

[momag46]

I wonder. Is it possible that the green button locks the flap in the closed position, thereby sealing the tank and preventing fumes getting out through the filler point, and limiting petrol escaping in the event of an accident?

No - with the button pushed in or left out and the car locked, the flap is free to open.

Everything above adds up to a load of sense. The bit about heat and expansion also makes sense, the plastic pan I use for the lawnmower comes up like a balloon - now I use super unleaded in the mower, as in the Xantia, it goes like crazy. I get the lawn cut in half the time but leave skid marks where it's dragged me along

Maurice