Dear Jochen,
I’m not surprised that the two columns of figures are different form each other. One is measuring temperature of a good radiator and good conductor which is attached to body of hot metal and the other measures an insulator which is in free air. Given that we know the temperature might vary by a range of 6’c anyway over a 60 second time period, I wouldn’t be able to diagnose anything by a variation within those bounds.
If you look at these threads
then what you need to check is whether the thermostat matches the housing well enough that it can block off the bypass route when hot. You’ll need a ruler and a bath of hot water.
At idle, the exhaust gas temperature (“EGT”) on a car like this can be in the order of up to 400’c and this heats up the cylinder head which cools via radiation, conduction elsewhere as well as by the constant flow of water. At idle you have a minimum of water flow and airflow is also generally low. It is not uncommon to see the temperature continue to rise by up to 10’c post shutdown because there is no forced coolant circulation. For example, lLooking at another datalog, after 2 minutes of idling, the water temperture rises 2’c from 86’c to 88’c as the EGTs read 360-385’c. Once the engine is switched off, over the next 70 seconds, the residual heat measured by the EGT sensors drops down to ~110-115’c but the coolant temperature rises to 91’c.
If your temperature figures are reliable, then it suggests that there may not be enough cooling going on when water flow is at its lowest level. This might be because the radiator and fan capacity is too low given their condition, or it may be that the thermostat isn’t fully blocking off the bypass so you are not using the full cooling capacity which ought to be available to you.
There is no guarantee that any new thermostats will perform any better than the old ones - I’d test them and compare them against each other before reassembly.
kind regards
Marek