CLK gas tank

[piersbc]

This is the second time in a month that this has happened....My fuel gauge reads low fuel...I've driven 280 miles and when I go to fill up I only can put in 8 gallons and the fuel pump lever clicks off...like if the tank was actually full. I then try to put more in but the tank seems full. The fuel gauge says there is only 1/2 of a tank and 8 gallons is 1/2 of what I usually put in it....So, is there an air pocket in the tank or a plugged vent line that would cause this???