On my 1982 300TD (wagon) with about 250k mi, I recently flushed the cooling system and replaced the green coolant it came with with the proper MBZ stuff and replaced the thermostat with a new tested unit and carefully installed it facing the correct way. after driving it around town for a while it burped and I toped it up. Well, I just got back from a long trip and it is misbehaving. On the highway it is running hotter than it used to, around 95C and in the mountains it will run right below 100C. running the heater cools it off by about 5 degrees. The upper radiator hose gets nice and firm. Normally I would think air in the system but I would have thought it would be worked out by now.
The wierd part is that there is a consistent yet intermitant leak of coolant comming out of the radiator cap (or the bypass hose to the aux coolant resevoir?) while running. There is a nice spray pattern of dried coolant radiating from the cap area of the radiator. I have replaced the cap and no change! I also noticed that when hot the auxillary resevoir is almost totally full and stays that way even when it cools down until I open the radiator cap and then it all drains back into the radiator. It is clear that the system is overpressurizing but I cant figure it out. what would cause the coolant resevoir to act this way? Thanks in advance