The gas cap of 300E has two extensions in the inside. If someone does not place the cap properly before twisting it to lock it, the extensions will rotate out of place and the cap will lock but not fully. You will smell gas all the time.

To fix it you have to rotate back the parts out of place.
The first time it happens it takes some time to rotate back the mislaigned part, but soon you realize that a s/driver and pliers are enough. Hope it helps.
Regards,

Juan