So i replaced my thermostat and whenever i run the car for about 30min the temperature starts rising and then drops out of nowhere and it goes back to normal, how come?
the temperature starts rising and then drops out of nowhere and it goes back to normal
The temperature should go up until the engine reaches operating temperature then level off and stay there. It should not go higher and then come back down. If you're noticing a fluctuation as I just described, then you likely have either air in your cooling system (as described by @justin-shepherd) or you could have a bad (new)thermostat.
Members can be more helpful if you include your year, make, model, transmission type & mileage.
that's the thermostat opening. The hot coolant starts flowing through the radiator and is cooled down. That's what it's for.