1984 300D with around 200,000 miles.

This past summer after doing a coolant system flush to try and reduce temps I also replaced the thermostat. The maden voyage after the work the thermostat failed to open and the coolant temp rose to 100C. That was on the way to work. I removed the thermostat for the drive home. I tested the operation of the thermostat on the bench using a heat gun and a Fluke Multi Meter with a temp probe. It opened as advertised at around 80C. I should say also that I calibrated the gauge sending unit with a Fluke dry well calibration tool. It is accurate.

I drove around until now without the thermostat. As would be expected the car takes a long time to warm up and now that it's gotten colder the heater function has suffered.

I reinstalled thermostat #1 thinking that maybe I had not purged all the air from the system. Same result. The temp gauge reads 90-100C and the lower radiator hose is cool to the touch. I have the thermostat installed with the disk facing aft as shown on the dieselgiant page. That direction also makes logical sense to me. The bulb of the thermostat is towards the hot coolant.

Still suspecting thermostat #1 I bought thermostat #2 and again made every attempt I know off to purge all air from the system. Again, same result.

I have no idea how to procede.

Phil