I learned a trick about 20 years ago on a BMW 2002 I had which had an external tstat ... have used it on every tstat I have installed since then - I and several other BMWCCA members went through dozens prior to finding ones that actually operated properly - someone came up with this idea and it appeared to work then.

I drill 3-4 tiny holes around the perimeter - the portion of the tstat that is thin (the part that moves) .. while I don't KNOW if this had made a difference since then in the 10-15 tstats I have replaced or simply removed and then reinstalled, I have not had a tstat that didn't appear to operate properly since then.

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George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg