First, I wanted to say thank you to Rex who posted in 2017. I benefited greatly from his post and other posts from Benz world, so I wanted to make sure I passed along my own update.

Problem: Noticed a gas smell in the cabin. Dissipated after a few days so wasn't sure if it was an issue with my car or another car parked in the garage of my apartment. Filled my gas tank a month later and noticed the gas smell again and noticed a puddle of gas below the driver's side seat (mid-rear section on the driver's side). I could see that the cover underneath the car was stained with gasoline and gasoline was dripping slowly from a small circular opening in the cover. The staining is easiest to see at night with a flashlight.

Solution: The fuel openings (one on each side) are underneath the back seat. Both were cracked and you could see 1 inch of fuel pooling on top of the covers. After speaking with my mechanic, the pooling on top of the covers slowly drips out beneath the vehicle so if you replace cover and seal, you should theoretically eliminate both the cabin smell and the dripping beneath, presuming no other problems with damaged fuel line or leaking gas tank. The fuel cover cracking seems to be a pervasive known design issue and had been subject to recall in certain 2010-2011 C-class models so try replacing the parts below before you spend money on replacing fuel pump or gas tank. The parts guy also said it is unlikely the fuel pump if my car has been running and gas gauge has been functioning properly. I had to replace the following:

- Closing Cover (Product ID: 204-470-04-38) RIGHT PASSENGER SIDE ~$55 - Fuel Pump Tank Seal (Product ID: 211-471-01-79-64) x 2, RIGHT PASSENGER SIDE, LEFT DRIVER'S SIDE ~$33 x 2 - Fuel Filter Unit (Product ID: 221-470-17-90) LEFT DRIVER'S SIDE ~$127 - Wasn't available right away from dealer parts store so suggest ordering online or calling ahead to your mechanic to make sure he has this part. Also you may be able to shop around your for your parts and get them 10-25% off. My mechanic suggested I use OEM parts as they are higher quality.

Many people have only had an issue with a leak on the REAR "Right" Passenger side. On that side, you only need to replace the seal ring and cover. If you have a leak on the REAR "Left" Driver's side, you need to replace the Fuel Filter Unit too. I replaced the seal rings on both sides. The total cost of repair was about ~$500 (~$250 parts + 2.5 hours of labor ~$250)

If you are handy, here is a tutorial on how to replace the closing cover and seal ring on the PASSENGER SIDE. - https://www.youtube.com/watch?v=yiO7OBN7S_g - https://www.youtube.com/watch?v=I-6rRazzeUY&t=28s

Here are additional links I found useful and informative: https://www.benzworld.org/threads/fuel-leak-smell-when-fuel-tank-is-full.2969506/ https://mbworld.org/forums/c-class-w204/719294-fuel-leak-when-tank-full-w204.html https://www.yourmechanic.com/estimates/mercedes-benz/c300/fuel-is-leaking-from-gas-tank-inspection https://www.bhbenz.com/parts/order-parts?path=%2Fsearch%3Fsearch_str%3D204-470-04-38

I ended up getting my car fixed at G&N Motors MBZ Certified Mercedes-Benz in Los Angeles, CA. George did a great job, took 2 hours, was very fair. He had me come in same day I called and fit me in. He also happened to have the fuel filter part even though the dealership didn't have it in stock.