Just in case....
I have a 300 E ('86)with a MB gas cap. I found that the fumes were unbearable at times and that the source was the protruding stems in the gas cap (they were out of place).
If you check the cap you will see two stems (can't think of another name); they move and get out of joint so to speak. You have to slide them back in place with plyers. It has happened to me several times because of the way that they are inserted (improperly) by the guy at the gas station. Hope it helps