Well Engatwork, that’s the problem – it doesn’t move much at all. When started from dead cold in the morning, it will rise during warm-up in a normal manner to about 85 and then it stays there for maybe 1 or 2 miles. Then as heat soak becomes more pronounced, it rises to between 97 & 100 and stays there no matter what the load or speed is. The only exception is if I boot it going up a long hill - then it will go up to just above 100.
I suppose that many will say not to worry about it, since it is stable within a narrow range & is not at the upper end of the scale. BUT this is not normal, and after effectively replacing every conceivable part of the cooling system that could influence this, I am stumped. And I like to delude myself into thinking that I’m a fairly competent DIY’er.
The only thing that I can think of is the thermostat is bad, but like I said, I did test it in a pan of water, and it started opening at about 180 (on my F cooking thermometer) – right on spec. I also raised the water temp to boiling, and it does open the aperture significantly more, although I have nothing to really compare it to, as far as knowing how far it is supposed to open. What are the chances of receiving 2 bad thermostats in a row from different suppliers & manufacturers? My original is gone (trashed it) the first replacement was a Wahler which did not open until it was hotter than 180, the second one is from the MBZ dealer, and is an Original Equipment unit.
BTW, I boiled up both the Wahler & the Dealer unit when I brought the dealer one home; this gave me a view of their comparative function, or in this case, dysfunction.
Thanks for the help!