W126 - Normal hissing or tank purge issue?

[tinypanzer]

I'm just curious - I'm getting a lot of hissing (pressure not vacuum) from the fuel cap when I remove it and the tank is near empty. This is on my 560SEL.


I'm just wondering at what point do I suspect something is amiss? Should I suspect some sort of purge/evap valve? Never really dug to far into the W126 vapor recovery system. How normal is this, and does anybody know of a decent way to test it?


Thanks much!

[Oracle12345]

I dont think it has a canister evap or purge valve. you see it on newer cars that had to meet stricter emission standards. It has the cis fuel injection. I get hissing when i open the fuel cap on any gas car. you are releasing the pressure in the tank. Whenever you do a fuel system service on a car you remove the gas cap so the pressure in the tank doesnt bulge the tank

[big dog 2]

I think your ride is just pissed at you! LOL!

[big dog 2]

Seriously, mine does that too on occasion! I was wondering about that too!

[hookedon210s]

The W126 cars have internal vent valves at the top of the fuel tank that control venting to the charcoal canister. Over time these valves jam in the closed position due to deposits from the fuel or condensation in the tank which prevents the proper venting of the tank. On a warm day the pressure can get quite high. To my knowledge there is no way to remedy this short of replacing the tank or somehow adding a vent line to the tank and installing the external vacuum/pressure valve. I had my 380SE tank professionally cleaned with no resolution of the overpressure issue. No bad effects to the car yet. Mark

[Oracle12345]

Originally Posted by hookedon210s

The W126 cars have internal vent valves at the top of the fuel tank that control venting to the charcoal canister. Over time these valves jam in the closed position due to deposits from the fuel or condensation in the tank which prevents the proper venting of the tank. On a warm day the pressure can get quite high. To my knowledge there is no way to remedy this short of replacing the tank or somehow adding a vent line to the tank and installing the external vacuum/pressure valve. I had my 380SE tank professionally cleaned with no resolution of the overpressure issue. No bad effects to the car yet. Mark

there is a vent line/valve on any gas tank and yes when its warm out the pressure on the gas tank is higher than when its cold out for obivous reasons.

this pressure is the reason why you dont overfill a gas tank. the owners manual listed capacity isnt 100% its a 90% to compensate for the pressure

[hookedon210s]

It's not overfilling that is the problem---it's gummed up/corroded internal vent valves. Mark

[tinypanzer]

So the valve is inside the tank and no way to replace it short of getting a new tank or bodging on a secondary vent?

That sux.


Hmmmmm........

[hookedon210s]

So the valve is inside the tank and no way to replace it short of getting a new tank or bodging on a secondary vent?

That sux.

Based on the MB workshop manual I have showing the vent system in the tank that is correct. Mark

[Oracle12345]

Originally Posted by hookedon210s

Based on the MB workshop manual I have showing the vent system in the tank that is correct. Mark

the vent valve is not located in the tank....only thing in the tank is the splash bowl and in screen filter....im looking at wis now....are you sure you are looking at right manual....

I was wrong in my initial post about not having a charcoal canister, its located in the engine compartment

[hookedon210s]

You are correct---there is a vent valve on the exterior of the tank. However, there is also a "vent system, comprising a collecting tray and a tube system" together with "additional check vessels" located inside the tank. See page 47.7 IV-030/3 of the M116.96 and M117.96 engine service manual for a diagram (I don't have WIS so I have to refer to the paper manual). In my experience it is this internal contraption that suffers blockage due to corrosion/gunk and can't, to my knowledge, be cleaned. Mark

[Oracle12345]

Originally Posted by hookedon210s

You are correct---there is a vent valve on the exterior of the tank. However, there is also a "vent system, comprising a collecting tray and a tube system" together with "additional check vessels" located inside the tank. See page 47.7 IV-030/3 of the M116.96 and M117.96 engine service manual for a diagram (I don't have WIS so I have to refer to the paper manual). In my experience it is this internal contraption that suffers blockage due to corrosion/gunk and can't, to my knowledge, be cleaned. Mark

fuel tank

[hookedon210s]

Here's the picture of what I'm referring to. Does WIS show this system or its elimination? Mark

[Oracle12345]

Originally Posted by hookedon210s

Here's the picture of what I'm referring to. Does WIS show this system or its elimination? Mark

thats for the early w126's but for the orginal poster the file i posted is correct. they made changes to the change over the years

[hookedon210s]

Originally Posted by Oracle12345

thats for the early w126's but for the orginal poster the file i posted is correct. they made changes to the change over the years

I hope you are correct for the poster's sake. The change that your picture described was the addition of an additional separating chamber in the splash bowl located in in the sump of the tank to stop gurgling as a result of vapor being returned with return fuel. The original tanks also had a splash bowl in the sump to provide a steady fuel flow when cornering with low fuel. The piping located at the inside top of the fuel tank is to provide a method to separate the fuel in the tank from the fuel vapors that had to be vented above the design pressure. All MB tanks from other models of the era had external expansion tanks to catch overflow and separate the liquid from the vapor. I seriously doubt that pipe 71 in the later fuel tank diagram exits the tank from the top without some mechanism to separate liquid from vapor. Of course this could be definitively demonstrated if tinypanzer would cut his tank apart for us Mark