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.

The stat in a benz as Kerry stated is a bypass stat.

Like a switch on a train track if it is closed the coolant circulates back into the block instead of to the radiator. Open diverts it to the radiator.

Take out the stat and the coolant can go either back to the block or to the radiator so taking the path of least resistance most of it will go back into the block so the car will run hot when it is hot outside and cool when it is cool outside.

You have to fill the system and start the car up and let it heat enough to open the stat, only then will all the coolant be released into all parts of the cooling system, ONce you get to that point you will be able to top it off most likely and get it full. It is good to check a third time though after running it for a while.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

If the weather was better, I'd have this done by now - the trouble with not having a garage...

I happened to mention overheating to someone who turned out to be a professional mechanic, and he said one reason for steam into the cabin (my very first "overheating event" while on the freeway) could be that the engine block cracked and exhaust gases put enough pressure into the cooling system to force coolant out of the connections between the heater core and the supply lines, and since I haven't gotten it that hot again, there is no active leak from the core (i.e. normal operating pressures won't cause this).

I didn't have much time to speak to him, but a cracked block would also explain why I can't fix my overheating problem. If so, it's amazing how well it runs with such a major "injury"! It would also mean I will have to sell the Brown Bomber as a parts car.

P.S. - Has anyone ever used a block sealant you put into the cooling system and run with only water?

The blocks are extremely rugged. To crack one you would about have to let it freeze. (Which is a possibility if you use that sealant with no antifreeze.)

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

U can purchase/borrow a little hand pump that connects to where the radiator cap/reserve tank cap goes. Hand pump it up, and see if it holds pressure. It needs to hold pressure. If block is cracked or U have a leak it won't hold pressure.

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1982 300SD

Oh, I doubt I would ever have a radiator freeze here, especially not *this* winter (barely below freezing for any one night)!

It was a brief conversation, so I think he was giving me one possibility for the amount of pressure needed to force steam into the cabin but not damage the heater core to the point of leakage - now that I think of it, another that comes to mind is a blown head gasket.

So this would show a leak, but how could I tell where the leak is? I just realized it could also be the head gasket instead of the block, but wouldn't know how to tell the difference.

Put in the "Block and gasket" sealant and refilled a couple times after more idling, and now the temp gauge doesn't go past 90 and I have hot air from the vents! I'll keep the stuff in for a couple fill-ups before going with 50:50 coolant.

A drive around town ran fine, but now I see the charging light at idle when cold (not really there when it gets heated up) - checked ground wires, regulator brushes and all the connections to make sure I wasn't missing anything, but it does charge after driving, so no worries.

any updates?

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Haven't been driving longer than a few miles lately because I needed to replace the fuel filter, but I will be putting in some freeway miles this week and see how the old boy holds up.