Please don’t think I’m being negative here Mr Mark Mate!
Your 75C <> 85C quick test findings sounds perfectly normal for your newly purchased aftermarket thermostat.
As best as I can recall the Dayco is labelled as 75C ?
All 3 x OEM thermostats (76.5/82/88) I trialled extensively (pot boiling & truck running) all displayed very similar opening & closing characteristics.
At said/labelled temperatures they commence opening ever so slightly.
Within 10C they all complete their contraction to fully open.
(Now it’s your radiators/fan turn to perform)
On the downside/cooling off peddle say downhill neither would fully close until some 5C below their labelled setting.
Relatively if your new Dayco is said 75C you will surely find fluctuations at the least between 70C <>85C and most likely over summer up to 105C if the rest is in order?
I’ll never personally use an OEM 76.5C thermostat ever again in Victoria with at best 20C fluctuations and at worst found 60C fluctuations due to their ever flowing tropical variant jiggle pins completely missing :-(
Please do be careful changing too many things at once as it may cloud your results and find one good thing could override another good thing underway
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