I replaced the line from the expansion tank to the overflow (in the right front wheel well) 2 weeks ago. It had been disconnected since we got the car, and I assumed it was only to catch the spills if we overheated. I was surprised to notice that I went from a normal operating temp of just over 90c to just over 80c with no other changes and it stayed there through our hot spell last week. Another example that these cars are made up of whole systems.
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)