Fuel filler breathing

I've done a search and haven't been able to find anyone talking of similar things happening, which probably means I'm not explaining myself well, so please bear with me.

Yesterday I got out to my landy after work to find a small amount of fuel on the floor. It was leaking out of the filler cap. I opened filler cap to find fuel right up to the top. I did fill up a few days ago so the tank was fairly full - only done about 15 miles on it since - but i fill up till the pump clicks off, so usually you can see a little fuel in the filler hose, but nowhere near the top!

Started engine, didn't really make much difference, drove a bit, it seemed to have gone down a little (i stopped a few times and took the fuel filler cap off).

When i got home i checked the level again and it'd gone down.

My first thought is there's air building up due to hotter weather, and pushing fuel out. There's a breather for this, which from a visual inspection seems to be working.

Is the fuel cap supposed to be vented (black with lock, suggests it should be) but I can't see any hole etc on the cap. It's on a 200tdi, but that's definitely not to say it's original.

Any ideas where to look? Bearing in mind it's been fine since I've had the vehicle. Maybe i've avoided hotter weather by having a less full tank in the past...

If the caps vented it doesn't vent to the top of the cap but over the seal on the inside. Some have a vent pipe off the top of the filler neck

Mine has the venting pipe in the filler, this goes from the filler pipe to the tank. Is there not something from the filler cap out?

Wouldn't that let water in when wading though? You can get waterproof caps which just omit the lock, I've got one and never had any problems.
I have read on here about someone having this problem before but I can't remember the outcome. If you can find the old thread it might help

I've had a search and wasn't able to find anything.

I also do (very occasionally) get a little woosh of air when opening the fuel cap, but thinking about it that shouldn't happen if it was venting properly, right?

If it is the venting that's the issue I see two solutions:

1) new fuel filler cap

2) add a t piece to the breather tube, and route to engine bay where I'm hoping at some point to extend the axle and gear box breathers. I'm guessing there'll be an issue in doing this (mixing fuel tank and axle/gearbox breathers) as that solution would be too perfect

I don't think the fuel filler is totally water or airtight otherwise there would be no point in the breather coming from the tank. I can't imagine the cap would be the problem, maybe its worthwhile checking the breather that comes from the tank and ends at the filler mouth and seeing if that's blocked. Either that or there's a problem as to why your fuel is expanding so much so that it creates a sort of vacuum until you open the cap.
You could always buy a new cap and see if it makes any difference but as I said mines is the waterproof version and I've never had a build up of pressure

I'll take a look. I am aware that taking the breather pipe off the tank end is nay on impossible without removing the fuel tank. This is something I want to replace soon, as the thing that works out the level of fuel is broke (it randomly changes between full, empty, and any point inbetween)and also the guard as a hole rusted in it

My mums Audi used to do it on a long journey, all of a sudden it would lose all power so we would pull over open the cap and a big 'whoosh' of air would come out and all would be fine.
I think it's hot diesel returning from the pump to the tank expanding the air and air locking the lines. Maybe something similar to yours?

MR BFG said:

My mums Audi used to do it on a long journey, all of a sudden it would lose all power so we would pull over open the cap and a big 'whoosh' of air would come out and all would be fine.
I think it's hot diesel returning from the pump to the tank expanding the air and air locking the lines. Maybe something similar to yours?

Click to expand...

dont think so as this is after it'd been sat all day, and it would start fine.

I got back to it today and wasnt as high (been awful weather here this am though) and i fiddled with breather pipe and it went down, so im wondering if its kinked or something. It's difficult to see, but doesnt seem out wrong

MR BFG said:

My mums Audi used to do it on a long journey, all of a sudden it would lose all power so we would pull over open the cap and a big 'whoosh' of air would come out and all would be fine.
I think it's hot diesel returning from the pump to the tank expanding the air and air locking the lines. Maybe something similar to yours?

Click to expand...

That sounds more like a vacumn caused by fuel level dropping and air not getting in......the 'whoosh' being air being sucked in