I browsed the diesel-engine forum for thermostat threads but found none that would describe how my car behaves.
The water temp gauge works pretty much as expected during summertime. It heats slowly to around 88c (to my knowledge, correctly working dti thermostat should open at 92c) and stays around there no matter what I do.
Come winter, and it makes no sense how, according to the gauge, the water behaves. If I do regular in-town driving; small gears, idling etc. but generally have my fuel consumption well above 10l/h, it will never heat above the first two grouped lines. The blowers give hot air if I so choose. If I take the highway and start driving around 80km/h (meaning consumption of around 5l/h) it will somewhat quickly rise to 88c. Immediately if I take off the highway and roll to an intersection, the temp will have fallen below 80c and in the next intersection it's again around the first two grouped lines.
As my logic would dictate that a thermostat stuck open would cause the engine to run cold during low consumption driving where the intake air is extremely cold, I don't see how this could be the case.
Then again, a thermostat stuck close would overheat during the summer this can't be the case either.
The readings are regular in a way that it always works the same.
If it makes difference, the car was imported from middle-european country to Finland. A friend of mine told that the thermostats might have different opening/closing temperatures in other countries.
Is thermostat the only part that can cause a problem like this or are there other possibilities? I will not remove the casing and do suspension-in-boiling-water-test.