I am at a loss on how to fix this problem on my 85 300SD. All the temperature mentioned is based on the dash gage.

I little background first. About 30000 miles ago, after doing a citric acid flush, I replaced my water pump and thermostat. The car would run consistently in the low to mid 85s. This winter I replaced the thermostat again as the car would take a long time to reach 80 degrees or for the heater to blow hot.

Since replacing the thermostat, the car will heat up to the high 90s but never quite reach 100. The top radiator hose gets hot so I figure the new thermostat is working. The only time the temperature drops is when I have the heater on high. The temperature will drop to the low 90s when the blower is on low and to the mid 80s when the blower is on high. The temperature would drop in 30 to 60 seconds.

Today I replaced the radiator. There has been no difference in the running temperature or the temperature dropping when the heater is on. The only thing I can think of now is that the temperature sensor is bad. I was thinking of getting an IR gun to confirm the temperature at the sensor. Does anyone else have any ideas?

Thanks

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1985 300SD