Hello jpmaynard
Suggest:
#1. Replace coolant temp sensor.
#2. Install aftermarket temp sensor and gauge to verify temperature.
#3. Check your cooling fan clutch.
#4. Test the electric cooling fan for function.
#5. Have a compression test done.
#6. Have a leak down test done.
#7. Don't let them fool you, #5. and #6. test for different things.

__________________
ASE Master Mechanic

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D

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