Bought a 2009 328xi n51 sulev. The gauge has always read 1/2 tank and would run out of gas when on empty. I would get to gas station and and it would only take 6 to 7 gallons and start running out of tank. Turns out the the vent hose on the bottom of the lid on the right side of the tank was not connected when whoever changed the fuel pump put the lid back on. I pulled the lid off and found the vent hose in the top back of the opening and attached to the lid. Note at this point gauge was showing 1/2 tank and the right side was full of gas about 1/4” from the opening in the tank. Put it all back together and went to the gas station and was able to put 9.7 more gallons in and now gauge reads full. If this vent line is not connected you can’t fill the the left (drivers side) of the tank because the fuel from the right side runs up the vent line and starts coming out of the filler neck where the gas cap is. All this time I thought the sending unit in the sealed portion (left) was bad and would have to replace the whole tank $$$$. Hope this helps.