This needs to be a sticky.. comes up every week..
Most of the problem is caused by the 'mystery marks' on the guage.. people thinking they are 50c or 60c.
when in fact they are 80c (lower mark) and 100c (higher mark).. 90c in the centre..
They should have been marked..
The guage is non-linear with the central 20c.. from 80c to 100c electronically 'expanded' to make it more readable.
Hence the 'mystery' marks.
Bugger all about it in the handbook..
Maybe there should have been a green and red section on the guage.. like a lot of other cars.
Thermostats are designed to 'fail safe'.. stick open.. an overcooled engine is better than an overheating one.
The cooling system is pressurised so doesn't boil to well over 100c.. usually about 105c ish.. thats why the coolant tank pressure cap has to be in good nick..
A low pressure = low boiling point = Hot engine.
Chuck in a partially blocked radiator or two.. The fact that ph1 twinnys ..ph2 twinnys and v6 all run at different temps. and the 'christmas cracker' guages arn't exactly precision instruments and everybodys confused.
Stay cool! (but not too cool)