This last weekend it looked like a Mercedes service centre at my house. Used the LiquiVac and changed the oil on:
1990 190E 2.6
Since my dad isn't allowed to pump, I had to do all four myself! Hard work. Guess what! On all four cars, the oil filler cap was incredibly tight. Three different engines, mileages varying from 55K on the E300 to 300K on the 190E. All tight.
I would guess that buying a new cap would solve the problem temporarily.
1998 C230 "Black Betty"