You need to get up to 80 or so to prevent excessive wear. The normal 617 thermostat starts to open before 83 and should be open there. I believe for a winter thermostat they use a 92 degree on that is used with gassers to lower exhaust emissions, but never lived where its real cold so I leave the standard one in place all year round.
You may not have a thermostat in fact. Or it might be stuck open either way you need to get it replaced.
I had a thermostat (in fact its still on my '79 300TD) that my mechanic friend drilled a hole in it to bypass the thermo flap in it. He said it burped the engine. I always back-fill the head and engine to prevent having a burping issue with any of my Benz's but I went along with his great modification to an old thermostat I let him sell me
I have seen new (stock) Behr thermostats with a small burp hole and a bb in it, so I figured it was OK even though he made the hole pretty large IMO. That car always ran cooler than my other turbodiesels. I thought one reason was because it has a three row radiator in it (special order and unfortunately I can't find another one like it!) but perhaps also because of the larger hole in the thermostat. Since that is my hauling wagon and I like to take long trips across the desert and in mountainous places, I left it in.
If it gets real cold with any diesel, you can place a piece of cardboard across most of the radiator opening. In the 123's is simple, raise the hood stick a thin piece of cardboard between the front of the radiator and the AC condensor. I used to do that for short trips to get the car warmed up. Doesn't hurt a thing. Older Big rigs have a series of blinds that open and close with a bowden cable to keep the cold from cooling their oil too much in the winter where chill factors are severe. Diesels need to get warmed up good to run properly. 70 degrees isn't real cold but it isn't the right operating temp either.
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 295530
1983 300D 243280
1985 300TD 217300
1987 300D 260200