Ok, perhaps mods can sticky this for all the 03-04 g35's with FAN CLUTCH setup.
First questions that need to be asked in the event of the overheating problem with the 2003-2004 G35 sedan
1. does it have a fan clutch?
if so, when does the car overheat?
A.) After driving on the road/highway and getting off the highway
B.) In bumper to bumper traffic
If (a) then you must check for proper function of the fan clutch
if (b) you must check for air in system.
First step is to test the functionality and worthiness of the fan clutch itself.
How to check?
(there is NO set method to check a fan clutch just takes experience)
one way to test it which is STRONGLY advised to exercise caution
is to grab a news paper and make it into a Strong rigid ROLL. once you've had the car turned up to standard operating temperature is to open the hood with the engine on and SLOWLY attempt to stop the fan. If your fan stops, toss it! If the paper just gets shredded and the fan does NOT slow down or make an effort to stop it's good.
if the fan is bad replace it. refill the radiator and proceed to proper bleeding instructions per Infiniti FSM.
If the fan is good then you may have air within the system.
If you have air you need to properly purge the system. One tool that I have found to work phenomenally well is NAPA spill free funnel kit part number 77-4000
With this tool you'll end your troubles quicker and safer.
If your trouble continues you must change your thermostat and re-bleed the air out of the cooling system.
one day to bleed the car quicker and more effectively is to disconnect the front 16inch auxillary fan from the car. Once you've done that you have to be more alert of the temps as you will not have that added protective layer that Infiniti created for the engine.
the fan kicks on at 203 degrees at 50% power and at 215 degrees it shifts on to 100% power.
The water begins to boil over at 210 degree's and radically boils over after that.
You'll disconnect the auxiliary fan attempt to overheat the car by revving the engine to 1,500 to 2,000 rpm. once you've achieved the boiling effect turn off the car allow fluid to come to a calm and then refill the bucket asap. Reconnect the auxiliary fan turn on the car and immediately fill the bucket so no air is introduced into the engine all in the mean while keeping an eye on the temperature sensor and while having the heater on full blast 90degrees.
within 10 minutes you can bleed the entire engine.
if you continue to have overheating issues you may have a warped head and if thats the case you need to rebuild your upper motor or just buy a newer / used motor.
You can test for this by purchasing a exhaust leak detector fluid thats poured into the radiator and if it turns a certain color it's bad.