Doru, thinking outside the box.....try this.
If you think your T-Stat is stuck open try pinching off the top radiator hose, not completely just maybe half to three quarters of the way. Try using vise grips or a C clamp with some pieces of wood on both sides so you don't damage the hose.
This should cut down the coolant flow and possibly make the engine heat up quicker.
You're only driving 12 km to work so the engine should not overheat but keep an eye on the temp gauge. If it heats up much quicker then your T-Stat probably is bad and needs replacing.
2000 540/6 Slightly tweeked...everywhere