Runs from expansion tank to overfow reservior in right fender well. Was broken off (old, brittle) at expansion tank. Removed right front tire and cowling from wheel well to expose overflow reservior. Replaced hose with one from FastLane. There was no sign of leakage prior, and I only did it to clean up the engine bay but the operating temp has not gone above 84 since, and we've had weather up to 90f a few times. Did the hose in early march and that's the only thing I did at that time or since to the cooling system. I replaced the thermostat in December, and I'm pretty sure it's an 80c t-stat.
It doesn't look like the overflow would come into play unless the coolant gets too hot, but I would idle at up to 100c prior. Only thing I can figure is that the overflow helps with pressurizing the system and closes the loop. Half hour job tops, and that's for a novice DIYer.
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)