I would review what M.B.Doc had to say in the following thread. He discusses issues with a warm up regulator. Those became obsolete along the way and I'm not sure if they were still around when your MB rolled of the assembly line.
http://www.peachparts.com/shopforum/showthread.php?t=122013&highlight=cold
If your MB is not so equipped, I would consider:
a. thermostat - running too cold
b. ECT - engine coolant temp sensor. If defective, the ECT could be sending a "warm motor" signal early in the game, thereby keeping things too lean.