SSShhhh when I open the fuel tank?

[Olivier]

Hi,
when the car is in the sun , I know, its not often here, and I open the tank cap first thing before starting the car, the tank is a little pressurised due to the fuel expending as it warm up. Then a Ssshhhh sound can be heard as the air goes out.
Is the breather working both ways? To let air in to replace the fuel used and to let the air out when the fuel expend, or its just to let the air in?
Thank you.
Oh, if this is both ways, I need to look at my breather then, where would that be located on a 1999 E300TD?
Cheers.
Olivier

 

[corned]

It's not fermentation going on in there, then?

 

[Olivier]

Might be something still frying in there

 

[h17n dj]

If any one can shed some light on this as I also get this on my W126 500SEL but never tested it when its been in the sun. everytime i go to the petrol station and open the filler cap it goes "shhhhhh"

 

[ringway]

Did you get confused with the engine decoke?

 

[dsan]

My SEL does the same, often when there is a quarter tank left of fuel. Not a big issue

 

[horgantrevor]

most cars if not all do this as above nothing too worry about

 

[Monkeylegend]

If any one can shed some light on this as I also get this on my W126 500SEL but never tested it when its been in the sun. everytime i go to the petrol station and open the filler cap it goes "shhhhhh"

I think is your car saying to you quietly," Shhhhh this is going to hurt "

 

[coalville126]

Could be vacuum in the tank normally caused by a partially blocked tank breather on a W126 and older models, not sure about the newer ones with ECU controlled venting system which needs some vacuum to work properly.

Read up about this on US cars in the warmer states as I had this on my car. It can get so bad that the tank slowly collapses inwards, affecting the fuel guage readings.

 

[WOODYTHEWISE]

I would think that if the fuel cap did not make a seal the fuel would evaporate, plus anyone smoking in a carpark would be in mortal danger from the toxic flammable fumes

 

[MarkJunedd]

I have noticed this on a few cars i've owned. Normally happens when the tank is low and the weather is warm.

 

[mauno]

check with a match if it does not blow then just air

 

[martyz]

MY 201 does it every time I open the cap,my 202 very rarely does it,and have asked this question many times to people that should know,got the same answer,some do some don't,no problem

 

[ianrandom]

Again, not sure about newer cars, but I know on 126's its meant to do that.
The tank is pressurised or vacuumed to a certain degree. There's a little valve that controls it above the rear axle area.

It can become blocked, I don't think its that common. My tank became noticeably over pressurized or vacuumed at different times, the tank shrank itself smaller and I had to replace it. Then the second tank did the same! The lines to the charcoal canister up front appear to be blocked, more expense than its worth to replace, so I replaced the valve and left it free to breathe, not connected to those lines.

I wouldn't worry unless the petrol cap becomes hard to remove at times, or is very pressurised when you open it. It was pretty noticeable.

 

[Dblock]

On my other car (non merc) you need to hear the shhh noise or the tank isnt sealing properly.

 

[mejermy]

+1 for coalville and ianrandom. I also am on replacement tank number 2. I took the opportunity to remove the charcoal filter when I had to go inside the wing recently to replace a rusted bonnet hinge. If you do nothing for a year or two I fear your tank will collapse.

 

[Olivier]

Thanks all, Seems common then.
The tank isn't going to collapse on my case as its pressurised.
Happy weekend to all
Olivier

 

[johnsco]

Both my E300TDs have done this ... Whether the sun is shining or not.

I don't think that diesels have charcoal cannisters .... Do they ?