Good point Brian.
We use coolant to raise the boiling point of water and we use pressure caps to raise the boiling point of water so we won't have a boil over. However, the temps I was talking about (and most others) only exceeded the boiling point of water by 5 degrees c or so - not much of a concern since the water can't possibly boil away after what we've done to it. AND, MB says that up to 120c is OK. We just don't like to see our cars run hot, despite the fact that diesels seem to run better when hot!

After my second thermostat, yesterday I drove home in the same traffic as before, ac on as before with the outside temps even a bit warmer than before. I had got 105c with the old thermostat, about 92 with no thermostat. Yesterday I ran at no more than 98. I'm happy with that especially after some hard pulling after the traffic jam that showed the same 98, about what happened with no thermostat. It makes me think that the cooling system has the capacity. Before, the thermostat was choking it (and may be still a bit, but I'm tired of buying thermostats). All this was done after replacing the water pump.

After more consideration I decided to replace the pressure cap, despite the fact that I never got any pressure in the radiator since coolant won't boil at 105. I found this online regarding someone who spoke to an experienced MB mechanic about pressure caps:

He says that the pressure of the
system has a much greater effect on cooling than just outright flow. In other
words, removing your thermostat can actually lower the cooling systems ability
to carry away heat due to the reduced pressure and lowered boiling point.
Apparantly, on the engine side of the t-stat, the pressure can be quite high,
like 20 to 30 lbs due to the water pump building head pressure against the
t-stat. This helps keep the coolant from boiling and creating spots where it
can't carry heat away because it's now a vapor instead of a liquid. I had no
idea. Makes perfect sense, but I had never thought about it and just assumed
that flow was the most important thing.

__________________
1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer