Had the same problem with my G35x, one day for no reason after a little bit of driving the temp needle was to the max. After it cooled off the radiator and reserve tank was full. I then replaced the thermostate and radiator cap myself and it was still running hot. Did a little research on the internet and found out that air bubbles are common in the g35 because the radiator cap and neck of the radiator is not the highest point in the cooling system. So if the radiator cap lets some air in the system you get an air pocket in the upper motor somewhere.
All I did to fix the issue was park my car on a hill so that the radiator cap was the highest point. Let the car run with the radiator cap off. I also unplugged the fans so that the coolant would circulate fast. Every once in a while I would see a big splash because and air bubble came out of the system. After about 20 min. I capped it up and its been fine ever since.

 

my car just overheated and first of all make sure before you change anything
check to see if you have coolant.
there is prolly air in your lines do a search how to bleed the lines to get all the air out

 

Same problem i concur with the above post you have bubbles in your system 90% chance and 10% its the thermostat. ( if your levels of fluid are adequate. ) There are multiple ways of getting the bubbles out. First you can test to see if you have the "bubble boom". When the car is cold start it turn on the ac at the 4th level so you can still hear whats going on and after a little ~1minute or so you should hear a sucking boom sound. It means you have an air bubble issue. Just remember when you are performing anything with bleeding the coolant system be sure to watch the temp and turn it off it starts to climb dangerously. It will cost you if you don't.

The recommended way is to use a proper bleeding tool but I have done it many times with the two ways below to g's.

Anyways two solutions, first make sure the car is cold and remove the radiator cap ( not reserve ) and turn the car on with premixed fluid ready with a funnel. Fill it as it sucks it in but if the temp rises fast or bubbles start stop and wait. Keep repeating this a few time and it should fix or you can do this.

second way to fix link.

 

I tried the method abruss suggested above - on a hill, radiator cap off. Not much happened for about 3-4 minutes then all of a sudden the coolant started pouring out from the radiator opening. I shut the car off immediately, let it cool for a few hours and then topped off the coolant level, pouring it directly into the radiator. Not sure if that did anything - my commute is too short for the overheating issue to occur.

Would a flush-and-fill at a local garage purge the air from the system as well? Or would I be better off letting the dealer handle that? I talked to my service advisor and he said that would likely be the first thing that'd they would try.

 

Sure, I think a Coolant Flush would work as well. And Sorry, but I forgot about that it overfilled on me as well. What I did was put a large funnel in the top of the radiator. The coolant would rise a little then some bubbles would come out. Again, sorry I forgot and left that out, it was about a year ago when I did this.

 

It's the thermostat. It's not opening and closing properly. starts off with hi temperature reading when sitting in traffic. Then the temp returns to normal when the car is moving. Its a ticking time bomb... One day while driving the temp rockets to hot and... game over!

 

I'm going to the dealer on Tuesday (won't be driving it all weekend) - I don't want to take any chances trying to self-diagnose this. Thanks to everybody for their suggestions. I'll let you guys know what the outcome is.

 

Just a quick update - brought her into the dealer this morning and just got a call with an update. Apparently neither of the fans were working right. One fan was working only on the "high side" as my SA put it, and the secondary fan was not working at all. They are replacing the motors in each fan. They are also going to bleed the cooling system. Fortunately they are making a bit of an accomodation since my warranty recently ran out, so I'm not getting tagged for the entire $650+ bill.

Anyhow, again thanks for all the suggestions - just glad I got it fixed before I toasted my engine!

 

Glad they are fixing it for you.

Seems like the fans fail more than anything else on these cars. I have had Nissan thermostats fail (in closed position) before but not on this car.

Part of everyone's oil change check should be to check the coolant level. We are a bit more prone to doing it here since it gets into the hundred and teens temperature range for most of July and August.