Just purchased a 2008 G35 for my daughter, upon test driving it seemed everything was OK. After driving it around for three days, noticed that temperature gauge spiked after driving around for about 45 minutes. Temperature outside was 105 and while driving temperature was right in the middle of the gauge, but when I stopped to get something to eat temperature spiked to Hot position on gauge. I noticed that one of the electric fans was not working, so I replaced the entire cooling fan assembly and fan control module. Temperature gauge is still spiking even though new fans are working now. I have not replaced thermostat yet, but I guess this will be my next option.

Any other ideas?

 

Simple stuff first.

Try changing your coolant.

 

Pressure test the coolant system, including the cap, to see if it's even holding pressure.