Engine not warming up

[sylvainfr]

Hi there, I’ve been having coolant issues with my n52 3si.
A few months ago, I replaced all the hoses with one of the silicon kits. Since that day, my car takes forever to warm up.

Because I had overheating issues, I swapped the pump, thermostat, and sensors.

I can't understand what could be wrong; I scooped the thermostat multiple times, and it is closed when the car is cold.

What I noticed was that when it was colder, the needle was slowing moving up, but as soon as I turned the heater on, I could see it going back down.

Any idea on what I should start looking for?

Thanks.