Condensation on Fuel Tank Cap

[Ralph Burnette]

I have a 1993 300SD (w140) with 255,000 miles on it.

I could not get the fuel cap off the tank the other day
due to rust on the inside of the fuel cap. I replaced the
cap but later saw condensation on the inside of the new
cap. The fuel tank was 1/4 full at the time.

Does anyone have any ideas on what may be causing this?
I have had the car for a number of years and never had this
problem come up.

Thanks.