280SL's have a recirculating system for petrol fumes. Located on one sidewall of the trunk, the earleir model was a fairly small black plastic box, later cars had a larger, flat box. They have lines going in and out of the box. One of the lines is a vent that goes through a hole on the trunlk floor to allow excess vapor to escape.
If the vent line is blocked, vapor can build up on the fuel tank especially and cause back pressure. If the pressure is high enough, a rush of gas fumes will burst from the tank fill pipe when the cap is removed especially in warm weather.
A clogged vent line might be putting back pressure on the fuel in the tank causing the fuel to seek an outlet although I have never observed liquid fuel comng out due to this condition.
Richard M, NYC
