Well, more proof I don't have experience as a MB home mechanic! I just assumed it was like others, where the coolant only gets to the radiator when the block has reached a certain temperature (about 175 degrees, if I remember the stat's rating). So where does the coolant go *before* the stat pops open?

As for the hose, when your coolant is so hot that your overflow tubing is venting steam, there should be *some* heat in the bottom hose - the radiator itself won't get the coolant down to ambient temps as the hot fluids run through it, and yet the lower hose feels untouched by all the heat going on around it (other than some contact conduction at the ends).

I'll put the stat back tomorrow, and top up the coolant as much as I can (but it's only water for now - weather in the 60's and barely 1 mile of driving don't justify antifreeze!) Any other hints would be greatly appreciated.