i have a K8400
and have some temperature issues
the temperature drops from 230 to 200 degrees because the fan turns on
So what is your problem?
When you like to cool down your coffee you will blow on it.
But I think you would like to have a stable temperature. Please have in mind that this is different to a stable temperature reading because of the critical and widely variable thermal coupling of your temperature sensor and the heater block.
You have mounted a thermal sensor within a drilling hole, which needs to be oversized due to tolerances. The temperature sensor is isolated by a glass net from the heating block, in this case a good electrical isolation and a good thermal isolation too is given. Now you blow on this assembly with the fan, cooling down the whole temperature sensor and the thick wiring towards the extruder PCB. Which temperature do you expect at the temperature sensor?
You have an unknown and widely variable interface between the heating block and the temperature sensor with an unpredictable temperature drop. This can lead to a melting of the PEEK isolator (approx. 330°C) even for temperature settings and readings around 200°C (see several topics in this forum).
That’s a small excursion on the temperature reading. Let us assume, that this have a minor impact, then you will have a similar critical interface for the heater cartridge. If the tolerances are too large, you will have only a small contact area for heating and the heating becomes inefficient. And when switching on the fan you need a lot of more power
These both conditions can be improved by using thermal paste. But be aware, that most of the available thermal conductive pastes are usable up to 150°C only. You need a non-conductive paste which can be used at higher temperatures. They are quite rare and more expensive, but I would recommend this in any case.
The third reason for this issue: you may suffering a poor setting of the temperature control parameters of the PID controller. Here you can try to optimize the PID settings for your hotend to get a better temperature stabilization (PID Autotune, M303). However, with the fan switched on or off the power demand of the hotend is changing dramatically and the implemented control mechanism needs some time to compensate this event. I have observed temperature changes up to 15°C in such cases with the original firmware.
Finally I recommend to have the fan always on on a low level (e.g. 30%). By this setting you will cool the PEEK isolator and the PTFE tube to avoid massive filament cloggings, the additional power demand for the hotend can be provided by an assembly with sufficient good thermal junctions.