How much pressure in fuel tank?

[BHap]

I have been getting some pretty good "whooshes" when I open the gas cap to refuel my car after driving for a while....  I have a '70 US 280SL with the evaporator tank in the trunk with the vapor return line to the engine...  Reading from the service manual they talk about the pressure being .012 - .018 kp/cm^2 for the pressure release valve to open sending vapor to the atmosphere...  does anyone know what this would equal in psi?  how much pressure should the tank have max?  how much vacuum from the crankcase?  Does it matter which side of the filler neck the two plastic lines going from the plastic tank are on?  Any information or suggested "tests" are appreciated...

[Benz Dr.]

I think you're talking about less than .5 psi so that all the pressure is vented off. There's something wrong if you have pressure inside your full tank.
The line that goes to the front and connects to the engine is under some vacuum but not that much. The engine actually produces enough blow by pressure that most of the vacuum is equalized. When you let off on the throttle from higher engine speeds there's fairly high vacuum in the manifold and this would be a time where any venting on the fuel tank could happen.


Daniel G Caron

[knirk]

BHap,
I just renewed my plastic lines with rubber lines due to a leak. I don't remember which of the two that was connected to which side, but I don't think it matters. The only line that is important to be connected correctly is the overpressure line that vents the fuel overboard through the trunk floor.

By the way, I have a question:
When my tank is near full, fuel is seeping out of the fuel filler cap. I can see stains on the crome below. My gas cap does not have a rubber gasget, which I believe it shoul. When I put it on it's metal towards metal. Can anybody confirm this gasket?

Per G. Birkeland
69 280 sl aut 834
Norway

[BHap]

there should definitely be a gasket on the fuel cap....  i have seen both cork and rubber....  i had this same problem with a brand new MB locking cap on a brand new fuel tank....  my solution was a second rubber gasket on the fuel cap (from a bmw - of all things) - now it is tight and a little hard to put on - but until i find a slightly thicker gasket - it doesn't leak...

[Cees Klumper]

I have a cork gasket on my relatively new MB chrome locking gas cap. It does not leak. There was an incorrect (vented) cap on when I bought the car, matte finish, that did leak. A gas cap should never leak / if it does, with the proper gasket in place, there may be a deformation of the filler neck that it fits onto.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[TheEngineer]

Yesterday I went by my friendly junk yard and-among other things-got myself a spare gas cap from a BMW 325. These gas caps are marked: "Ohne Lüftung-Blau" meaning not ventilated. It's a perfect fit. Cost me five dollars. The BMW part number is 16111184718 if you want to buy it new.

1969 280SL,Signal Red, A/T, P/S, A/C, Both tops.    Ret. engr.