If it were my car I would test the 02 sensor and the water temp sensor... If it is out of range, thinking its colder than it is, it can over enrich all the time.

But Im not really familiar with your cars fuel injection & emmission systems...(97 C230 right)... I don't even know if you have a water temp sensor.

Try the snif test after it has fully warmed up though to just get a rough idea if it seems to be burning too much fuel.
Wish I had more to offer.
Good luck.

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html