Jetta JR Overheating: Causes and Solutions

Hello everyone,
After replacing the faulty coolant level sensor in the expansion tank (8 euros for an original VW part), I noticed that the temperature gauge no longer shows a reading in the middle, but instead reads three-quarters.
I just replaced the thermostat and flushed the cooling system, and everything looked normal, and nothing was clogged or anything like that.

Question:
Could it be that it came from that puddle?
'For the past few tankfuls, I've been adding 15 liters of kerosene (Aldi, €0.75 per liter) to my diesel. The car runs much more smoothly with it than without, and I almost look forward to adding more kerosene each time because it makes a noticeable difference in performance.'
So far, I haven't been able to find any information online that links 'Pöl fahren' (presumably a specific activity or term) with a temperature increase.

Can anyone comment on this?

Regards,
Hendrik.

Are you sure that the Schaetz-Eisen gauge is displaying the correct readings for that Golf 2 (Jetta)?

There are two temperature sensors on the water flange. Simply swap the connections, and if it then displays the temperature "correctly," then the sensor is faulty.

If it still shows too much, then either both sensors are broken or the device is actually getting too hot.

I still can't guess it, sorry.
m;