i have a 2003 g35 sedan, recently had a crack in my radiator, so i replaced it i noticed my temp gauge spiking and my heater stopped working, so i replaced my thermostat. was working fine for a little but once again spiked. took it to a mechanic and they replaced the thermostat again he says "even good ones go bad" .. well that didnt solve the problem he ran pressure test on cylinder heads and tried eveything, still overheating... i figured there was air pockets so we bleed the system with car on an incline radiator cap off and even unscrewed bleeder valve.. solved problem for two days, and now i am here for help... anyone?

You have air in your system still.

Bleeding the VQ engine is kind of a PITA, most mechanics don't do a good job. It takes about 1.5 hrs to do it right.

Engine off.

#1) open radiator cap and bleeder screw. Pour coolant into the radiator until it overflows out of the radiator OR the bleeder screw. If it comes out of the radiator first put the rad cap on and then squeeze the upper radiator hose until it fills to the top of the bleeder valve.

#2) Run engine until the engine gets to the normal operating temperature and the T-stat opens. Rev engine to 3000 rpms and hold for 10 sec. Do this 3 times paying attention to the temp gauge. Don't let it climb within the last 2 notches of the top.

#3) Turn off car. Open the bleeder valve VERY SLOWLY (using #4 philips head tip) so that you can hear the air escape. When no air is coming out and only coolant then you need to close the valve.

#4) Wait 15 min and repeat the bleeder valve open slowly procedure.

#5) Let the car cool and repeat procedure until no air escapes.

Do this and you will remove the air from the coolant lines and you will get heat into your car again.

Good luck, take your time. Normally only takes 2-3 times for me. I open/close the bleeder to let air escape as much as i can.

ok thanks i will have to try again.

I got the same problem, I had a crack radiator, so I replace it with a new. The car is still over heating, I ask around and they say I need to replace the thermostat I did that still over heating, So then I was told that the coolant sensor might be bad, I replace that. I tried bleeding the system still over heating. I called Orlando Infiniti and the service person there told me it was my cooling fan. I don't understand because it turns on that means it work right? What else could it be
So I replaced the radiator $142.00 I replaced the thermostat $26.00 I replaced the coolant sensor $25.00 If i replace the cooling fan that will be $278.00.
How does air get into the system in the first place?

Like Will said, keep bleeding the system a coouple times. only if youc ant fix it then you'll have cause to suspect fan.

is it turning visibly slow or something?

Check the FSM's here in the FAQ.
I followed the procedure a couple of times when I did a drain & fill on my coupe and runs fine.