I don't have a pic for this test but with the stat in the housing and a stick pushing the stat against the housing to simulate the spring, I poured water into the piston. The water drained out pretty quick out of those 4 small holes in the piston. the only path out of there. Water is much thinner than oil viscosity wise and drains faster than oil but with the stat totally closed, the path from the stat to the oil cooler is not totally sealed.

Has anyone performed the FSM oil temp stat test and got the results as described (abrupt rise in oil cooler temp when the oil reaches 95 to 110 C depending which temp stat you have)?

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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