The thermostat on my 1985 300D turbo stuck open and the temp sat at about 50. I replace the thermostat and for 250 miles of highway driving it stuck right at 95. Toward the end of that trip (the last 20 miles or so) it moved up to 105. Why would an engine that runs stone cold without a thermostat not hit the pre set temperature and stay there? The air conditioning compressor is disconnected. Has to be the new thermostat? How much trouble do you have with new thermostats? Do I have to much time on my hands? My father used to call it motor worry. You are driving home late at night and the whole family is asleep and by the time you get there you have a rod knock, bad universal joints, and four bad wheel bearings, but it's all better in the morning.

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Dan Stadt
'85 300D turbo
'79 300SD
'73 220D

If you were still at highway speed the last 20 minutes, and the temp just went up to 105, I would suspect a problem somewhere, maybe with the new thermostat. If it went to 105 after you pulled off the highway, and it eventually returned to 95, it should be ok.

I assume you used the correct replacement thermostat.

On my 240D, the thermostat is on the intake side of the water flow, and the temp sensor is on the head. Therefore, you can see temperature differences between the thermostat temp setting and the operating temp on the guage.

If always been told not to worry unless the temp guage goes to the red.

Possiblilty one, you got a faulty thermostat. Any part can be bad right out of the box. (I had a new, Gates serpentine belt fail and strand me 50 miles after replacement! The new-new Gates has gone 10,000 miles and is still good!)

Possibility two, incorrect installation. Upside-down so it doesn't open correctly? (I don't know if it's possible on this motor, but it was possible on some domestic engines.) Gasket sealer partially blocking the thermostat?

Possibility three, blocked or collapsed suction hose reducing circulation. Coincidence, not related to thermostat replacement?

Possibility four, incorrectly calibrated temperature gage/sending unit, so that 95 is 80 and 105 is 90.

One, two and three require attention soon, while four is the best since it can be fixed but isn't a real problem.

BCingU, Jim