I bought my 300D new in 1983 (over 200K miles) and never had a problem with the fuel cap until a couple of months ago. I noticed a diesel odor and discoloration on the fender. Checked the fuel cap and it felt firm but in reality only one tab was engaged. Reseated and cleaned the car - no problem. Happened again this week. I don't know how much the mileage decreased, but I was clearly pumping diesel on the ground from the return line.