Thank you very much George and Hunter,
I had the t-stat replaced early this morning but before I could have a chance to read your advice, espcially paid attention on the little hole as orientation. I didn't remember where I had it faced. Is the positioning critical? Should I reopen it to make the change?
Other thing was if I have read your advice before the replacement, I would have the new belt covered more careful to avoid any possible splash, but I did pay attention for not to get coolant on the belt though. I used water to wash the coolant off the engine afterward. The old one got stretched badly may be related to the coolant splashed when the pump leaked.
I noticed the old t-stat did have a stretch mark on the bottom half of its shaft, indicating the hesitation of movement until higher temperture forced it to move further. (This was my theory of course).
One thing I found was during running, the gauge stopped at approximately 87 or 88 degrees mark, and would increase after 3 minutes or so stopped in traffic, then rose to about 100 and park there. Didn't go any higher. A great improvement I thought.
Since the temperature still risen a bit compare to what it was before the water pump replacement, I think the coolant mixture may contribute some more heat to it. I read a few threads about the coolant mixture here, that more antifreez content (or percentage) can cause the engine run hotter.
Anyway, I am happy for the cooler engine, and ready to add more freon because most of them escaped after the pressure got too high, I think ! ( I am going to learn another lesson on how to add freon a simple way next).
Thanks you very much folks, you guys have been the fantastic advisors. I love this forum, and have been learning a lot.
1994 C280, Red hot with cream interior 106K miles
2000 ML 320 Silver
1991 190E 2.3