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Replaced thermostat that was stuck open. Now my 2006 Nissan Xterra is overheating. Why?!

I’ve had a thermostat that was stuck open for a while now. Finally replaced it and now when I drive it it’s very very hot to overheating but not quite on the brink. And losing fluid out of my reservoir through the overflow spout. Is this just air in the system ? And if so will it work itself out or can I get the air out of it somehow ?

Are you sure you have the proper part or that you intalled it properly? there is a valve on them that needs to be in the upper position. I assume you bled the system after replacing the thermostat, and replaced the missing coolant.

I’m literally impossible to install it wrong, as it is 3-bolt asymmetrical pattern.
I would be much more concerned if the part was “right”.
On RockAuto it is clear that aftermarket thermostats are different from one another.
Some of them have some kind of additional “flow shaper” attachment, some are just clearly generic units put in specific body.
I endd up buying Nissan OEM unit and it had that “flow shaper” thing present, so I assume it is needed for the proper operations.
OEM part is not that expensive to compare to aftermarket ones to take any chances.

Yeah it wouldn’t go in any other way and it looked exact same as the other one. I did not bleed it when I refilled it. I filled the radiator and ran it. And filled the reservoir while it was running and some told me that was the way to let it suck it back into the system as it was getting the air out. Should I try to leave the radiator cap off and start it to let air out ? Not much experience with cooling systems

See below the response. Also I had to drain the entire system to replace the thermostat as it said to do. It was the whole housing and all

Check the very high end on the pipes what go into the firewall on the passenger side.
You will find a rubber cap, 1/4 inch internal diameter, pinched with a C-clamp to the pipes.
Let your engine cool down.Remove the cap and fill coolant until it flows from this bleeding port.
Close it, do not forget to put a C-clamp in place, ENJOY.

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Possible to do on a 2006 X trail-- however different engine than yours, also used two thermostats. Word to the wise if you have not replaced the original radiator cap do so asap. The seal on mine failed last summer and I lost a lot of coolant.

This worked for the most part. Still wants to jump up when I turn the heat off. But if it’s on it runs at perfect temp now. Thank you

Put your heater on the max temperature and let it run until “very hot” from the heater: it will push some of air pockets from there.
It may also take more than one drive-cycle to completely get all air pockets out. Keep adding fluid into expansion tank, don’t let it go dry or you will get more air trapped.
You might want to repeat that bleeder port trick on cold engine again to get as much air out as you can.
Good luck!