No pressure in coolant reservoir - '83 300D
 

I have an unusual problem in my '83 300D (W123) - I have no back pressure when I remove the cap to the coolant reservoir and the car runs slightly hot. It has 181K, and has had the reservoir replaced with factory Mercedes in the last 2 years. Running down the highway at 70 the temp runs between 100 and 110 degrees Celsius on the temp gauge, and it runs about the same idling. If I turn the heater on, it drops just a little (right below 100 degrees). I do not trust the temp gauge, it jumps around a little (I cleaned the terminal and sprayed contact OL on it), but I do know there should be pressure on the reservoir. There are no obvious signs of a leak anywhere and I haven't had to add water in the last year of ownership (the temperature gauge has alwayd run between 80 and 100 even in the winter). I also took a hand-pump pressure checker and can't get the system to hold pressure. The whole thing just doesn't add up, especially to the fact that I haven't had to add water yet the system doesn't have pressure. Any ideas guys?

hows the cap?

Cap was brand new with the reservoir - has the "magic" three pointed star on it so it must be good :D.

do you notice any white smoke out the tail pipe?

No white smoke out of the tail pipe.

replace thermostat?

Even though the resevoir is pretty new, it might have a crack in it. Have you pressure tested just the tank?