I don't think that is a valid test after looking at my stat. Those 4 vent holes will let the water through and will also let oil through to the cooler before the stat opens. So I don't understand the FSM test that says you should get a sudden rise in temp at the inlet of the oil cooler when the oil reaches the opening temp of the stat.

Edit: I mis spoke. Those 4 holes are the path of oil flow to the oil cooler. How are they sealed against the housing when the stat is closed? I'll have to take a closer look at the housing.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked