You may have a failed head gasket. If it is allowing compressed air from the engine to enter the cooling system, this can happen.
Do me a favor. When the engine is cold, remove the radiator cap and start the engine. See if coolant blows out. If it does, the head gasket is my first suspect. Also, check to see if there are air bubbles in the coolant while it is running.
Take a look through this link and let me know your thoughts. It discusses different things to check, one of which you already mentioned isn't happening (mixing of oil and coolant).
Let me know your thoughts. If you suspect it could be a head gasket, I recommend checking engine compression to see if there is a cylinder or two with lower compression.
Here is a link that explains how that is done:
Let me know what you find or if you have other questions.
Take care and Happy Thanksgiving.
Wednesday, November 23rd, 2022 AT 10:09 PM