Alright I'm posting this thread after many days of searching several different G35 forums. I have seen dozens of threads with 2003-2005 G35 owners complaining about their car overheating when outside temps are in the 90s and the A/C on when in stop and go traffic. I haven't found a good thread to answer this problem, but did see this solution mentioned once or twice. My G35 also had this problem and this is a free fix/starting point before taking the G to the dealership and blowing hundreds on a new radiator/condensor/compressor that may really not be the problem.
The fix which is outlined below is to take off the bumper and powerwash the condensor/radiator to restore the efficiency of the cooling system.
My 2003 with 130k miles G35 had these symptoms:
1. A/C would work great 98% of the time- would blow very cold with no problems.
2. 2% of the time (always when hot outside and in stop and go city traffic) the A/C would start blowing hot air and then the car would start to overheat. The temp guage would rise to just over 3/4 position.
3. The A/C would start to blow cold again once I could get the car moving to get airflow over the condensor and radiator.
4. NEVER would have any problems when driving on the freeway.
5. I confirmed that both the engine fan AND the electric fan mounted to the condensor are both working 100% normal. Meaning that the engine fan would not be stopped or slowed lightly touching some rolled up newspapers to the fan (this is to test that the fan clutch is not bad). The electric fan turns on once the A/C is turned on and stays on the whole time the A/C is on.
6. No radiator leaks or clogs. Coolant level fine and coolant in good condition.
First thing I did was to take the front bumper off the car so you can get full access to the entire surface area of the condensor.
Good front bumper removal DIY here- http://rupromenergo.ru/forums/v36-diy...r-removal.html
Remove the 3 bolts holding on the electric fan and disconnect the electric plug so you can completely remove the fan.
Remove both horns which are each mounted with 1 10mm bolt. Mark the wires on each horn so you know how to reconnect them correctly.
(Its hard to see here but with the fan and horns removed you have full access to almost the entire front area of the condensor)
Next, leaving the condensor in place and mounted on the car, I took a power-washer to it blowing from the front of the car and made sure to wash the entire surface area of the condensor making sure not to use too much pressure to damage the fins or tubes on the condensor.
(Fins run vertically on the condensor, Tubes run horizontally. DO NOT PUNCTURE any of the tubes or you will be in for a world of hurt and expense replacing the condensor. Be careful with the fins also as damaging them will reduce the efficiency of the condensor)
*Note- I don't actually own or have access to a power-washer so I drove the car (yes with the bumper and all off) to my local carwash at about 1am and spent about $5 in quarters for enough time to wash all the gunk out. I was AMAZED at how dirty the water was that was coming off the condensor. After doing this I could really see how the efficiency of the condensor would have been decreased because of the the gunk and bugs stuck in the fins and between the condensor and radiator.
EDIT: Some powerwashers spray at incredibly high psi. Be very careful not to use too much PSI if you do use a power washer. I took my car to a car wash to do the cleaning and even then the closest I ever got the tip of the sprayer to the condenser was about 12 inches. The risk of pushing small rocks further into the condenser and actually causing a puncture in one of the tubes is definitely a possibility. If you don't want to risk damaging your condenser an alternative method would be to drain and remove the radiator and spray the condenser out from back to front to ensure you push debris OUT of the condenser and not further into it. I was willing to risk the damage and I made sure to be very careful with spraying it out at the carwash.
I also cleaned with a small screwdriver at the very bottom of the condensor a small gap where leaves and all sorts of junk had gathered. The pic below shows how much crap I removed from this gap- and this was AFTER power-washing it all out!
While I had the car stripped down I took an hour to straighten many of the damaged fins on the condensor as best I could. Over time small rocks and bugs bend the fins down and interfere with the airflow through the condensor.
While I was at it I drained and refilled the radiator. My coolant really was fine, but while I had such easy access to the drain plug I couldn't resist some cheap extra maintenance. (Disclaimer- DON'T send your wife to get the coolant for you at Autozone. They will talk her in to the $20 gallon of coolant instead of the $8 gallon because "it works so much better than the other stuff". Thanks Autozone!)
I then put everything back together- making sure to test the horns and electric fan BEFORE I put the bumper back on.
After doing this I have had the car out in 95 degree temps idling for about 20 minutes absolutely parked and not once did the A/C start to blow hot or the car start to overheat. Total cost $5 carwash, $20 coolant (ouch) but optional. Much cheaper than anything they will do for you at Infiniti!