nestorb98, trust you installed an 87 degree C thermostat. As mentioned, sometimes new ones are faulty right out of the box. Try a Wahler 87 degree C thermostat, and be sure the vent hole is towards the firewall (at least that's what I recall as the correct orientation). Also, confirm the coolant expansion tank cap holds the proper pressure.

While you have the thermostat removed verify the coolant level is just below the thermostat housing flange. If not, add coolant mix before installing the thermostat.

When filling the radiator I use a short piece of clear hose temporarily attached to the upper radiator connection to ensure the radiator is completely full. Then remove the clear hose and install the upper radiator hose.

Until the problem is resolved check the coolant level in the expansion tank when the engine is cold. Add coolant mix as necessary. It's possible there is a pocket of air trapped in the cooling system.

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Fred Hoelzle