my 617 right now stays around 100C no matter what and I tend not to believe it either...FWIW I have a new thermostat in hand and am waiting to replace it.

is there a definitive way to test that temp readout?

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car