I have the same issue as the folks here in this thread. Temp goes right up to 100c, and stay right there like that's where it's supposed to be.

I am going to change the thermostat first. Then flush the system (PO has green coolant in there now). I guess I need to get an IR temp reader, any other way to verify that the temp gauge is accurate?

What else should I check along the way? I really don't want to replace the radiator...

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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