When to buy new car?

That is the million-dollar question, everyone must decide when the car has had enough, there is no set way to do this. You must look at the condition of the car and how you feel about it. If it is super clean and you have taken care of it, and you don't want car payments then I would keep it, (It has 140,000 on it so there is some life left in it), but if the repairs are getting too much to bear then it could be time to move on. It is up to you.

How does one calculate it assuming there is no sentimental value? Go against resale value versus car costs over a certain amount of time, or the value any car would offer financially against costs over a certain amount of time? Someone must of formulated this? Probably statistics on when people typically sell or donate what was their new car?

Does the check engine light indicate well if the engine is damaged when due to overheating? My light didn't go on. Just the heat meter. I wasn't sure if the check engine indicator wasn't working or something.

Have the tariffs hurt foreign car prices much? I'm worried about paying like 25% more for a new car.

Luck has a lot to do with it, sometimes people have nothing but problems with the car and then sell it, the people that buy it can have no problems whatsoever and say it is a greatest car ever, it is a machine. You need to do some research on the used car value and go from there, I don't know about the tariffs.