Thank you BenzMac. Sorry that I haven’t replied until now. I’ve been busy replacing my radiator amongst other things lately. Unfortunately, my car has been living up to my pseudonym here, Running Too Hot.
On my (new, old) 84 300D with almost 200,000 on her; the radiator was original and succumbed to the “neck crack” syndrome. Luckily, I caught it the instant it happened, so I didn’t fry the engine.
The (metal) bypass line was not plugged when I checked it. I now have a new radiator, new hoses, OEM pump, a dealer-supplied thermostat (plus a Wahler that was defective out of the box), a 40/60 mix of phosphate free antifreeze + Water Wetter, a perfectly functioning visco clutch, a fully functional auxiliary fan (which I discovered via the wiring diagram is NOT turned on by *any* engine temperature switch, ONLY the A/C receiver dryer switch!! W.T.H.?).
There is no air trapped in the cylinder head, since it is self-venting. The temperature gauge is reading accurately, checked multiple times with an infrared non-contact thermometer used on the engine block & cylinder head. The new (2nd) thermostat was checked in a pot of water, and it starts opening at 180 degrees. The system was flushed thoroughly – and it was clean to begin with; nothing dirty came out, the water was barely discolored at all.
After all this, the engine runs at 97- 99 degrees on the gauge, no matter what the load – light load, medium load, A/C on, A/C off, 60 degrees ambient, 90 degrees ambient, etc. You get the picture. Can you spell F-R-U-S-T-R-A-T-I-O-N? I guess that I should be satisfied that it is at least *stable* at 97-99 degrees, but I am going to try a THIRD thermostat. I can’t think of anything else. If the head gasket were leaking into the cooling system, I don’t believe that the cooling system would continue to hold pressure in overnight, which it does. Unless there is such a phenomena as a one-way leak (?!).
Sorry for the ranting. If you have any ideas, I would GREATLY appreciate them. I may post this as a separate question for the group if you think that would be wise.