I have a 2004 350z that I recently swapped a new engine in to. During the swap all sensors and components were replaced and the radiator and thermostat had been replaced about 6 months prior. The heater is hot at all times, even idle, so I know that there is no air in the system (spent multiple days making sure the car was bled completely) however after a long period of driving (half hour or so with heater full blast) the car begins to slightly overheat (happens quicker if the heater is off but doesn't get any hotter). Slightly meaning it isn't anything alarming as the coolant temp needle only goes a bit higher than operating temperature and hasn't gotten to the H mark on the dash at all but this is still weird to me. The car is full bolt on and tuned on 91 and has a Weapon R overflow, could it just be time to upgrade the radiator to an aftermarket one? It doesn't seem like any component has failed and the thermostat is opening as well as there are no leaks but after days of bleeding the coolant system the proper way (car angled up, spill free funnel, hold at 3,000 rpm for ten seconds blah blah blah) I took extra time because this was an engine imported from Japan that had been sitting for a while but I'm out of ideas as to what could be overheating it. For reference I did a crest run two days ago (1 hour drive) in 30-40 degree weather and had to stop twice to let the car cool down when it started to creep up in temperature but I can drive the car to campus (15 mins) no problem.

 

Figure it out?
Any luck?

 

Does your car's heater work? In other words, it you turn the interior car heat on, does the cabin warm?

 

To me it sounds like the thermostat may have failed / be failing. I know you said you changed it but that does happen.