C124 fuel tank pressure ?

[whirsk]

Hi Folks

After a spirited drive today (1990 300ce) I could hear 'gas' escaping from the fuel cap, when I opened the fuel cap there was quite a pressure release.Is this a common occurrence ?

 

[carat 3.6]

There should be no pressure in the tank, you may get a vacuum in there though.

 

[grober]

There is a fuel tank breather filter under the nearside wing hidden behind the wheel arch liner- its periodically evacuated by the inlet manifold vacuum. That said its frequently neglected [ out of sight out of mind] or the evacuation mechanism is faulty [ its via a solenoid valve in later models] If its choked then this might lead to fuel tank pressure build up.

 

[whirsk]

Thanks for the help folks.

 

[hotrodder]

http://www.w124performance.com/service/w124CD1/Program/Engine/LHIS/47-0030.pdf expalins how the positive and negative tank vent works. While 50 millibar is very little pressure in the grand scheme of things a fairly noticable pressure release (audible whooosh for a sec or 2) when opening the cap is normal as a fuel tank is a LOT bigger than say a soft drinks bottle...

 

[WDB124066]

I get a whoosh of air inwards I think, my regen valve [looks like item 52/1] is open too much of the time making the car run lean & hot, if I replace it and the charcoal filter is there anything else I should replace at the same time...

My understanding is the regen valve [52/1] sucks air and a little fuel through the filter using inlet manifold vacuum. The diagram hotrodder posted has everything going the other way, what's going on...??

Ahhh, just seen "a" comes back to RTFD; Read The Flippin Drawing...

 

[hotrodder]

Are you sure the whoosh is from a vacuum? It's fairly subtle but should be able to feel the cap get 'pushed' off as you release it. I can on mine (or pretty much anything with active tank venting) If your's is pulling a vacuum then the charcoal canister is the first suspect

IME a dead (stuck open) regen valve results in a high/hunting idle (effectively a vacuum leak) but that's the only noticable thing. Simple/quick test is to disconnect the hose from the regen valve and block it to seal the vacuum leak. Probably already aware but if not then 47-200 covers operation of the regen valve which can felt pulsing when it's working 47 Fuel system - M104

 

[WDB124066]

I am not sure if it is a vacuum or pressure release from the fuel tank, it does vary in magnitude quite a bit though. Have to look a little closer.


Looking for 52/1 physically, I'm guessing it is behind the wheel liner like the filter...??

Pays to check the whole system.

And yes there is quite some flow of air through the regen valve dropping vacuum and leaning the fuel mixture to the point where the car will increase in running temp considerably [and idle roughly] when it is faulty. Can see CO mixture on a exhaust gas analyser go up from a low value when you block off the entry port to the regen valve, as the mixture richens due to the vacuum leak being plugged.

Still looking for 52/1 on my 1996 cars...??

 

[hotrodder]

It's not something i've ever actively looked for/made note of as the only problems i've ever had with the EVAP system has been stuck/dead regen valves. The drawing suggests it's not gonna be particularly accessible/obvious, like you say removing the arch liner to get at the charcoal canister will reveal the hose to it while the cup seal itself looks to be positioned so that the back of the indicator hides it from the engine bay side. Assuming the drawing in the pdf is accurate for all years

 

[WDB124066]

Yes I agree following the line from the filter sounds promising. To the OP I think it important to look at all four parts of the system and the lines to make sure they are in good working order. It looks like there should not be much pressure or vacuum in the tank when the various components in this system are working correctly.

 

[ianrandom]

Yep, a little pressure/vacuum is OK, if it's in spec. On the W126 this is controlled by a very cheap part clipped onto the hose that goes away from the tank - vent valve, less than a tenner mine was.
In my case, the actual lines were blocked - I discovered this after my second bloody fuel tank started vacuuming itself smaller..!

So my vent valve is still controlling the tank pressure, but the other side of it now vents to air instead of going on up to the charcoal cannister to get fed back in to the throttle.

Maybe not the same but thought I'd share in case it helps you avoid changing your fuel tank.

 

[maska]

Hi Folks

After a spirited drive today (1990 300ce) I could hear 'gas' escaping from the fuel cap, when I opened the fuel cap there was quite a pressure release.Is this a common occurrence ?

Quite Natural, But You Don't Want A Vacuum. !!