Pbarratt1
New Member
Hi all just refueled and when i removed the filler cap there was quite a rush of air does this mean my fuel tank breather pipe is blocked or is this normal for a w220cdi
coupe deville
Active Member
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- 93 E320 coupe sportline facelift
There was always a whoosh when I removed the fuel cap, I wasn't sure whether it was sucking in or blowing out, so I drilled a very small hole in the cap,---------- sorted.
kth286
MB Enthusiast
you now need to buy a new cap as the fuel evaporation system allows for both a limited amount of pressure and vacuum in tank depending on various criteria at different times.
At certain times you will now have a leak from engine inlet manifold to the outside world via that hole in the cap.
At certain times you will now have a leak from engine inlet manifold to the outside world via that hole in the cap.
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grumpyoldgit
MB Enthusiast
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Pbarratt1
New Member
You are correct grumpy cdi's certainly are diesel
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Pbarratt1
New Member
No worrys but still none the wiser tho
grumpyoldgit
MB Enthusiast
Perfectly normal. Most cars have pressurised fuel tanks now, petrol or diesel. 
coupe deville
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- 93 E320 coupe sportline facelift
BLOODY BUGGAR Really ?
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Pbarratt1
New Member
Just reading on the net that my car may have a fuel tank vent valve is this correct as the vacuum was quite loud when the cap was removed not just a little hiss
hotrodder
Active Member
All diesel and petrol tanks are vented, they're only sheetmetal (or plastic these days) and can't take much in the way of pressure (or vacuum as the fuel is used). In the old days the vent was open, for environmental reasons they haven't been for quite a while and engines are forced to eat evaporative fumes. Dunno about a w220 but back in the w124 era the fuel filler cap served double duty as a relief valve in case the 'active' vent fails- have a look at the underside and you should see a spring... kinda, sorta like a rad cap but far less obvious. Like a rad cap it doesn't prevent pressure build up, just limits it
FWIW i make (among other things) fuel tanks. When leak testing them you can see them just begin to change shape with as little as 3psi in them in some cases (obviously depends on size, shape etc). 3psi might not sound like much but that's a fair chunk of air in the case of a 70 litre tank. Most of my stuff is for older cars that'd origionally have an open vent, i try and persuade people to buy a proper valve at the same time rather than run an open vent... the type i use allows air into the tank, opens at something like 1psi to vent pressure but won't pass liquids i.e. will shut in the case of a roll over.
FWIW i make (among other things) fuel tanks. When leak testing them you can see them just begin to change shape with as little as 3psi in them in some cases (obviously depends on size, shape etc). 3psi might not sound like much but that's a fair chunk of air in the case of a 70 litre tank. Most of my stuff is for older cars that'd origionally have an open vent, i try and persuade people to buy a proper valve at the same time rather than run an open vent... the type i use allows air into the tank, opens at something like 1psi to vent pressure but won't pass liquids i.e. will shut in the case of a roll over.
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Pbarratt1
New Member
Thanks hotrodder will get the garage to check if I have one of these valves or not and if not how I go about putting one on
