Bit of an odd one, but this is my go to place for random thoughts from a variety of inputs. Sorry!
My 2.0L HDi (2005) used to run at 90 degrees. No Question. Winter, Summer, Motorway or stuck in traffic going up hill.
The other day, i noticed it was running at 83 degrees (99% of people would even have noticed i guess)
I mentioned it to the mechanics when it went in for a service, and their thought was:
“whatever it is, it WILL be the Thermostat, and by the time we have diagnosed it youve paid for a ‘stat several times over, they are about a tenner”
The new stat and service didn’t “solve” it (or change anything), but seeing as its not a “problem” they didn’t seem to bothered or inclined to go rummaging around the cooling system
Now i have noticed that sitting in traffic, the temperature rises and rises (as you might expect). according to the garage the fan comes on when it should, so it isn’t a problem.
Except, it never used to do this. It used to sit at 90 degrees irrelevant.
It is Warmer weather i guess at the moment, the rad isnt taking “enough” degrees of temperature off the engine per cycle of coolant, and therefore the temperature can rise when it doesn’t have air flowing over it? In cooler months the rad can keep the coolant level down on its own?
Perhaps ive just answered my own question….