Hey, does anyone have any idea why the car has the difficulty to reach 90c no matter the weather conditions?

Have changed both first and second thermostats and have a paper behind the grill for less cooling and the car either doesn't reach the operating temperature or drops everytime I go down the hill. It seems like as soon as the car doesn't use any diesel the temperature starts dropping. When going uphill the temperature starts to rise slowly. I can go for 30min drive and still have 70c. Have had a few diesel cars before and know that in colder conditions diesel can take a longer time to reach operating temperature but at this point I can be driving in warm weather and still have 70/80c by the end of the drive. I bought the car 2 months ago so I can’t really tell if it’s always been like this, but I never owned a diesel car that wouldn’t be able to reach the operating temperature and stay there until shut down.

 

Hi! You didn't mention the year of car or mileage on it.
The coolant thermostat and EGR Cooler thermostat....are those the temp control parts you replaced on it?

Technically, those shouldn't open until specific temp parameters are met, allowing the engine to get up to normal temp as quickly as possible. Living in Norway, you likely see some very cold air temperatures, which can make it difficult for a TDI to warm up, especially if you've got some bad sensors on the car, which gets back to how old is this car/mileage on it?

I've just had to replace my own coolant thermostat at engine, as well as coolant temp sensors...car struggling to get warm, and was allowing temp to fall off if engine not under load, much like yours is, but mines the 3.0L TDI engine. What I found after swapping the thermostat is the temp sensor by radiator had failed, and that put cooling fans into a constant-run default cycle, even though it was frigid outside (0 to -10 F). There's usually a second temp sensor on Audi engines too, that sends the temp to the ECU, etc., so best to replace both on car, as they both eventually go bad, and relatively simple to change out.

In researching my own under-temp issues, I found that you need to look for any odd symptoms with car, such as very high speed cooling fans running while engine is on, very hot air from vents in car only, abnormal fluctuations or 'jumping' motion on the temp gauge in car...hot indicated, etc., as these all pertain to different sensor/part failures.

Not sure where you have the 'paper' located to block cold ambient air from reaching the heat exchangers/radiator, etc., but that is a great solution in a super cold climate region. You may need to revisit how yours is setup to gain better coverage/blockage, since when you coast you are losing heat so rapidly. Definitely be sure those radiator cooling fans aren't running crazily when they shouldn't be running. If your HVAC is in Auto mode, that generally means the car controls the A/C compressor and can run it at will unless you manually turn it off (A/C switch inside car, on HVAC controls). To test my cooling fans I turned off the HVAC system entirely, which takes it out of the control-feedback-loop for cooling fans. With engine cold, running, and HVAC off the cooling fans should be completely still/off. Mine were still running, which pointed to the coolant temp sensor next to radiator, which normally controls when they run, unless being overridden by the HVAC system telling them to run, etc.

After the coolant temp sensor was changed out, the cooling fans stopped running all the time (HVAC turned off). Since I changed the thermostat on front of engine block, my car now maintains normal operating temp once it reaches it. It is important to understand that thermostats are mechanical/thermodynamic, and react to the coolant temp flowing past them, so in extreme cold weather the temp could fall off before the thermostat can close/reduce the coolant flow.