I chased this one for weeks........and weeks..........
I run 160f thermostats in my SRT's.
There is some logic somewhere in the cars temp gauge system that refuses to allow it to run cooler than 85c desired.
If you set fan at 86c desired, it will cycle nicely between 85c and 89c.
If you set to 85 or less, as soon as that temp is exceeded, the fan will run all day long,
even till the thermo closes and cool water is coming out the rad outlet
I tried all sorts of things, but best fix I have so far is to run 2 ECT sensors.
Extra one goes on cyl head and is wired in parallel with OE.
This reduces resistance and makes the ECT appear to be about 20c hotter than it is.
So now 75c engine temp reads 95c on gauge which keeps fans happy.
Rejig ECT fuel modifiers to suit and all is good!
Fans work perfect and using laser temp gun, engine temps stay in solid 73-78c range.
Cooling system is up to the job too, never gets hot altho fan runs a bit more as you would expect.
Lower ECT does wonders for helping keep knock under control on 6.4's