That was my take also. The SDL is the first motor I've seen with a thermostat on the lower radiator hose. Seems like it would make for difficulties in water getting into all the necessary spots. On the upper mount, also makes sense, but seemingly less crucial. Pretty sure many or maybe most all of the cars I've owned had the upper mount thermostat with no bleed provision. I've read several times with the SDL/603 that when filling, be patient. Fill it up, wait a half hour and top it off again, do that for a while.

What you say about 'heater valve open during warm up' might be an issue. My heater is always full hot. I've had other issues that need attention so I've let that one go for now. I have, of course, heard of climate control issues. And I know a little about the monovalve, I put in one of the nototious aftermarket units in my SDL and remarkably saw an improvement. I think it was an earlier gen of aftermarket units, my parts guy said it had been on his shelf for about 15 years. What I don't know is the what and where of the valve that allows hot water into the heater core. Not sure if the monovalve is where that happens.

I was a bit spoiled with my years with the E30s. The heater on those cars is remarkably straight forward and reliable. You can get heat where you want it, when you want it.

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Te futueo et caballum tuum

1986 300SDL, 362K
1984 300D, 138K