I am no mechanic although your car "overheating" is the key word here in Diagnosing your issue.

You need to be sure if the car is overheating.

Because my car does not over heat at all. The car runs fine.

Your car overheating would bring up a concern with the thermostat.

get back to me.

 

Thanks for the quick reply. I replaced the thermostat yesterday, but the car is still overheating. They did a full coolant flush and ran the car for around 45 minutes trying to bleed the system, which seemed to stop the car from overheating, but it's still blowing cold air at idle. Should I take the car back to Mr. Lube? It seems like there's still air in the system.

 

I would make sure your radiator fan is coming on, I got a recall notice in the mail stating that the radiator fan in some G35s fail and the car overheats,

Also it could be a number of problems that are causing your car to over heat, the water pump can fail internally, the impeller fins on the water pump can sheer off do to rust and some fins can remain connected to the pump, so at a cruising speed or high speeds it can have enough power to push the coolant threw the system. but while at idle there is not enough flow to cool the engine or give heat.

Also you could have a clogged heater core, even though they did a cooling system flush it doesn't always get all the stuff built up in the heater core, so the heater core sometimes has to be flushed manually with a water hose or something.

so the basics look like they have been covered, thermostat replacement and cooling system flush, maybe try a new radiator cap also.

 

Thanks Libra. It doesn't sound like the water pump to me, because the car only overheats under load. Is it really difficult to unclog the heater core?

 

Well both heater hoses have to be removed, they go into the passenger side of the fire wall, I wouldnt say it's easy at all. but if your a DIY kinda person then it shouldnt be to bad then, you will need to replace about 1/2 gallon of cooant though because it will leak out, then you have to make sure all the air is out of the system after.

 

So far so good . my Heat is working after the 3rd flush/refill

Let's see how long it lasts.