Should the fuel tank of your Mercedes w124 230 ce presurise or is there a vent?

It is not pressurized. A breather valve at the fuel tank allows fuel vapor to flow out of the tank and be stored in a charcoal canister until it is drawn into the intake of the engine. It also allows fresh air into the tank to prevent a vacuum. The fuel cap is not vented, all air moves through the breather valve. Technically it may, at times, be pressurized or have a vacuum. However, a properly functioning vapor system should limit that to a max of .725psi, or a vacuum of .45in. Hg.