Oh, and if you have been feeding radiator seal/ cracked head sealer into the coolant system, I'm betting that's half of your problem...
That ***** blocks holes! Like water galleries ! Like the fine/small ones in the head and cores! One can of that crap can block a dirty heater core!
Have a look at the nav zd head gasket water ports/galleries... see how small they are compared to the "head and blocks" water galleries on either side of that gasket! Theres a reason for that also!
What do you think happens there when you add buckets of radiator sealant to a dirty coolant system...
You may have had a blocked coolant system at the start and not a cracked head before you added the tubs of sealant!
Did you test for a cracked head before playing superbleep mechanic and adding mountains of Apooh to your radiator lol
How many "grams" of "sealant/SOLID " have you fed it?
Lol, I'm sorry, but, where do people think it magically goes? Fills a crack then the rest magically disappears lol no, it settles in your system and causes issues for the poor bloke that buys it from the car yard
Me personally, I'd be buying 5lt of c.r.c and flushing it with that
That "additive" is a swine, great to get you home- but a swine to deal with afterwards! If you did have blocked cores, that crap has now settled over it like a steal blanket, no flush will remove it! Reverse flush may, but doubtful!
Buy the manual and have a look at the coolant direction and galleries! These stinking zd coolant systems are insane, if you have a faulty radiator cap it can cause the Thermostat not to open.. yup, there's that much pressure running that system it's nuts!
Why would you put the Thermostat in that position Nissan???
And, Yeah, stop using additives my man! Once maybe! But twice- you've got motor issues.. try a flush treatment- a few times, you may get lucky- or it will show her weaknesses, head or core!
Good luck..