I just ran the eng. up to operating temp. and the thermo. cover didn't leak. I added more coolant as she needed it. I am a little gunshy to try and remove the other 2 bolts. The engine is warmed up, but it's raining now, so it will have to wait. From what I can see, there would be enough of the bolt sticking out to grab with with a pair of vise-grips once the cover is removed. I guess a good shot of WD-40 might of helped.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member