Positive Fuel Tank Pressure, gas coming out of lid (W124/M104)

[John5788]

Hello,


I have a strange problem that I haven't seen before. When I open my gas cap, gas spills out of the hole like it is overfilled.


I recently filled up the tank at a gas station then parked the car after driving maybe 5 miles, nothing out of the ordinary. The next day, I noticed heavy smell of gasoline in the passenger cabin, so I opened the gas cap and a decent amount of gasoline came rushing out of the hole.






I wasn't planning to drive that car any time soon so I just put the gas cap back on and continued working on some other stuff. The next day, I checked the gas cap again, and gasoline spilled out. This happened yet again for a third time today.


I have not driven the car or even started the engine since I parked the car on Friday. Today is now Wednesday, so 5 days has elapsed since I last drove this car.


What is causing this overpressure and gasoline to come spilling out of the gas cap when I remove it? Sure the evap vent valve is probably broken and not letting excess pressure out, but unless a chemical reaction is happening inside my gas tank, why is there so much pressure being built up while the car is sitting idle on the street?

Edit: A video for reference: https://streamable.com/jk3dff

This video is from the 2nd time I opened the gas cap. I got the video recording on my phone started as soon as I felt pressure on the gas lid while removing it to check.