What prevents a Sidekick from our years(or any car) to reach 180F?

[zukitrek]

Hi Fix. This is just a question out of curiosity. Whenever your tell people when asking for your help in engine/idling problems you mention almost always that ECT must 180F. Id imagine if temp does not reach that number then some sort of situation in that matter may follow. What could prevent a car reaching that temp and why if not?

Regards!!