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Date registered: Sep 2008
Vehicle: 1975 450SL
Mentioned: 2 Post(s)
Quoted: 15 Post(s)
I see why you would question the water pump. that would be my next check, too. If the pump isn't pumping, the heat would stay in the head. Did the radiator get hot? If not, I would suspect the pump.
Just to say it because I've seen it happen: Did you put the thermostat in the correct way? The spring side should go toward the engine side. Experienced mechanics have gotten them in wrong, being distracted at a critical point, so it may be worth checking.
When you burped your cooling system, did you have the heater on? Something I didn't mention in my post, as I've bypassed my heater core due to leakage problems. Sometimes air from the core will get into the pump, and with nothing to "chew" on, it won't circulate.
Is there a small hole on the rim of the thermostat? If there is, it needs to be at the topmost position.
Did you ever get the bit about the oil not mixing worked out? there could be something there.
Just a bunch of ideas, from a long ways away.
Remember: It's not just a car. It's an Adventure.