1.2tsi Petrol 13plate Octavia.

I can't help but notice that there seems to be a lot of air pressure in my petrol tank.

It's like this if it's been 3 weeks since opening, or 1 day.

Does anyone else get this when removing the filler cap.

Both videos were taken on freezing days, so I can't really see how it is built up fuel vapours.

Is it actually a vacuum rather than pressure?

Also, the sloshing of the petrol will build up a bit over vapour pressure even when cold

First video looks like a lot of positive pressure.

Malfunction in the EVAP system I think.

Any recent work done on the engine?

Edited 15 January15 Jan by Breezy_Pete

At first I did expect a vacuum, obviously due to fuel usage, but it's definitely pressure, because it blows the cap off.

I've seen the possibility of an EVAP system fault already, of course it looks like the charcoal canister is hidden behind the rear wing somewhere, and not in the engine bay.

Which way do you think it might be:

Solenoid valve not opening to suck tank/canister vapours,

Or pressure being sent back to the From manifold to tank/canister

I've already located the draw from canister to intake manifold, so will remove that with engine running and see what happens.

I do get intermittently and randomly "lean off idle condition" fault code.

Thinking on it, I've already pressurerised the inlet manifold and found no leaks, so it can't be going backwards.

Although that was with engine off, so solenoid should have been closed.

Not sure how it could happen, to be honest. I guess there would have to be a one-way valve somewhere that isn't working. Not sure if the purge valve itself has any reverse flow prevention in its design.

Two things:

EVAP system - if any of the solanoids have failed, this will flag up as a fault on the car. Please get the car scanned via VCDS or ODBEleven

Fuel Cap - is this the original cap or is it a replacement cap? If replacement, then please try to source a genuine OE spec cap.

Yes, sounds like a evap system fault to me. I've not encountered mine, but they can have set rules about when to run and report on evap system check complete. So if you are not using the car much, or never fill the tank on cars gone by that can prevent the evap checks from running.

A full scan may help, there may be pending. Or there maybe a function to force check evap system.

It should be vacuum surely. Positive pressure should be purged to the charcoal canister.

The pressure is only building whilst driving, because I opened cap last night, then again this morning and there was no pressure.

I say pressure as it only seems to be about 0.3 bar, it seems like there may be a relieve value somewhere in the EVAP system (see bellow)

I was able to replicate the pressure in the tank by forcing air back into EVAP line, but it wouldn't go above 0.3 bar.

No fault code for the purge valve, and it does have an arrow on it suggesting that it may be one way.

Tested it and it doesn't pass air either way, but this was with the engine off, so it shouldn't anyway under that situation.

So at this point I'm concluding that either, the valve is operating correctly, but when opening allowing manifold pressure back to tank.

OR

The valve is never opening, so never venting petrol tank vapours. (But I believe this scenario should throw a fault code)

I'll have to take the car on a longer journey to see if I can get the value to open and repeat the tests.

Just thinking I haven't checked for a blockage in pipe from the valve to manifold, so will have to check that aswell.

Of course this happened while removing the pipe, Arrh why plastic😠 and not rubber.

So with the pipe repaired, and engine fully up to temperature.

At idle the purge valve solenoid is constantly being energised, rapidly fluctuating, and opening and closing (repeatedly) roughly twice a second.

This seems excessive?

Also interestingly with the pipe broken and open to the atmosphere, the tank still pressurised.