In my 1992 300D, coolant temp gauge usually reads 90 C. It went to 95 C last summer when A/C was operationg during city driving.
When A/C is not operating and the car is driving at 50 ~ 70 MPH steady speed, the gauge reads 85 C.

Because the manual says that 80 C is normal operating temperature, I asked about this to a dealer. The dealer tech said that that temperature range was OK, and it was not worth putting money due to this matter.

Although an indi (Hatch & Sons, MA, USA) said, about a year ago, my water pump and thermostat were leaking, other mechanics did not suggest replacement of these (a mechanic did pressure test and found no leak). Coolant level is quite steady.

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.