I changed my coolant several years ago and decided to change tstat for assurance. The replacement was a behr. Same identical one I replaced. The filling procedure was through the radiator hose manner. Drove the car and temp was about 95c. I did try to purge air but no result. I finally ordered another tstat and repeated. Almost the same result. Same manf same temp. I finally took the three tstats and did the thermometer hot water test and the two new tstats opened 10c higher than the old one. I cleaned that baby up and it is still in the car and doing about 75-80c. Test the tstat first.
Bud

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1987 300D Turbo, 175k mi., 1998 BMW 323i Convertible, 1997 F250 4x4 7.3L PSD