That is the check valve. Its open or closed.
You need the purge valve under the hood.
The purge valve only scavenges off the vapors. Its computer controlled. it just "sniffs" the vapors and allows them to burn off.
If it sticks open it will draw fuel into the canaster and eventually up to the engine. The engine will run erratic and stall. Not idle. It will be running rich.
Make sure its a Mercedes part!