I have a 03’ 350z and I just bought a thermostat directly from the dealership and replaced it, started the bleeding procedure but the thermostat seems to not open and fans just stay running loud. The lower hose going into the thermostat is cold while trying to burp the system.

 

Next time test the thermostat before installing boil water in a pan and put the thermostat in if it doesn't open its bad and it has happened to me also these thermostats are not the easiest to swap hope this helps

 

When the car is cold and you start it, do the fans come on right away? Why did you replace the thermostat? Was your gauge pegged hot? You probably have a bad temp sensor if the answer is yes to the first and third questions.

 

Never mind I just had to bleed the system several times and it worked now