I have a 04 350z 128xxx miles. I recently purchased the car and I noticed the car ran cold. The check engine light eventually came on P0128 thermostat. So I pulled the housing and noticed there was no thermostat. So I replaced it with a duralast one from Autozone. The car ran hot even after multiple attempts of bleeding the air out as I’ve read up on in this forum since it’s a complicated system. I even replaced it with a Beck Arnley one and Nissan oem one as also suggested on the forum to no avail. I pulled it out and it will run fine but cold no overheating. It will warm up if idling but go almost cold once driven. I know this can be problematic long term running the engine cold so I decided to take it to Firestone. So they tried a new thermostat to no avail also. They pressure checked the system and also told me radiator flows properly, water pump is circulating like it should, fans are working, no leaks, no suspect of a head gasket issue but they just can’t figure it out either and pulled the thermostat back out and suggested I speak to the person I bought it from which I don’t have his contact info and it was sold as is anyways. Any suggestions here?

 

Suggest using a block/coolant tester for th head gasket.

and then just using a nismo thermostat. and having a tune put on it to tolerate the lower temp.

 

 

This coolant system is notorious for being difficult (not impossible) to bleed air completely. Details can be found here if you search for a thread called "Burp". One thing that worked very well for me- There is a bleed valve on the piping just outside the heater core in the engine bay. Instead of unscrewing it a bit to let air escape, I removed it, installed a funnel and used it to pour coolant into. Literally priming the heater core directly, worked great in conjunction with normal radiator cap topping off. GL!