First, I apologize for starting yet another thread on this issue. I've read, and read, and read, and experimented, and switched parts, and I'm just at wit's end.

I did an engine swap after the original engine puked in this 84 300SD. First time I've ever done anything like this, but even working 100% alone, it's running driving just fine. Except for the coolant problem. I flushed it out twice with water. Even took the hose off the back end and flushed out the block. Lot of crap was in there. Got everything back together, and it wanted to heat up on me.

So, here's a list:

1. Radiator only has approx. 6000 miles on it. No cold spots, so I've ruled out that.

2. Water pump and thermostat were both replaced (not new, but the ones from the old engine, which had no cooling problems). Both had been recently replaced (within the last year or two).

3. I have hot heat, so apparently it's circulating through the heater core.

4. All hoses are hot, none cool.

5. After cutting the car off, I can hear gurgling from the center of the dash, where I assume the heater core is.

6. While running, the coolant level in the tank is rhythmically rising and falling.

7. There are no visible leaks. Anywhere.

8. My process for initially filling up the coolant was to fill the reservoir, remove the top radiator hose, fill the engine from the thermostat housing/upper hose neck, replace top hose on thermostat end, fill up the upper hose, attach it, and run the engine with the overflow tank cap off (while parked uphill).

9. Drove the car 12 miles today, and it ran below the normal temp (at approx. 70 degrees on the gauge). This was with the overflow tank cap loose. While sitting at the drive-thru at the credit union, idling for a few minutes, it crept up to the normal temperature that I'm used to (approx. 90 or thereabouts). Drove across the street to fill up on diesel fuel, cut the car off, and water runs all over the parking lot (from the overflow tank). I left, and it was still not running hot, but it gradually crept up on me as I neared home (eventually near redline by the time I found a good place to stop for a while). After about an hour of monkeying with it (let it cool down, loosened the top radiator hose enough to try to bleed some air out, added coolant) I got it home (again with the cap loose). The rest of the way home, about 8 miles or so, it ran about normal again. Just for kicks, I left it running when I got home, tightened the cap down, and loosened the upper radiator hose again, in hopes that I could bleed out some of the air that I've apparently been unable to do otherwise. It was hissing out pretty good, so I let it run a little while (it was still hanging right at 90), and came in to take a quick look at my email. I went back out, and it was REALLY hissing air (or steam, or whatever), and the temp was up to 100+.

Questions:

1. Is that a reasonable way to bleed it, loosening the upper radiator hose to use as a bleeder (I read it on the internet so it must be true )

2. Is it possible that having the thermostat out of the old engine, exposed to air, and then moved to the new engine, could have hurt the used thermostat? I heard that one from a friend of mine. Just throwing it out there.

3. I'm running water in it right now, as coolant was getting pretty expensive for experimental purposes. That shouldn't have anything to do with it, should it?

4. Should I just go ahead and shell out the cash for another new thermostat and water pump, or am I wasting my time?

5. Anybody have any ideas at all??

Down to my last cigarette. Going to town, but I'll be back on later.

Thank you in advance, and again, my apologies for starting ANOTHER cooling problem thread. I'm just so confused at this point, hearing so many different things. I just don't know where to turn or what to think.

__________________
1984 300SD, 729K "Ol' Blue"
1980 240D 4-spd manual 470K "The Ugly Duckling"
1988 Ford F250 7.3 diesel, aftermarket turbo
1987 300D, 380K
1982 300D, 276K
and many more MB's from the past!

Although most of my experience has been with the 616's, I've recently aquired an MB with a 617 engine. Fliped my out when I didn't see a radiator cap! Anyway, The cooling system functions as a closed circuit similar to the circulatory system in our bodies. I will build pressure and that is how it run effectively.
For cars with a radiator cap, there is a tester which is secured to the nect of the radiator fill and measure normal pressure. If memory serves it around 12 psi but I could be wrong. The hissing which you are hearing is from the pressure being released somewhere in the circuit. This indicates to me that something in the circuit is open.
Don't think it's a thermostat. If stuck closed the engine would over heat. If stuck open, the engine would never heat up with a constant flow of water throughout the cooling system.

Yes, I have some experience with the old 4-bangers too. My '80 240D has clutch issues right now (4-spd manual).

I'm figuring there's air in the system somewhere, but it's just figuring out how to get it purged. It's driving me up the wall... Seems like I've both problems. Either it's running too cold, or running too hot. It's like it has a mind of its own.

__________________
1984 300SD, 729K "Ol' Blue"
1980 240D 4-spd manual 470K "The Ugly Duckling"
1988 Ford F250 7.3 diesel, aftermarket turbo
1987 300D, 380K
1982 300D, 276K
and many more MB's from the past!

All of the work to the cooling system is proper and the refill procedure is proper.

You cannot get the results of the coolant blowing itself all over the pavement from "air in the system".

Find a suitable tester to see if there are combustion gases in the coolant.

This situation seems far more serious than "air".