Engine Overheating Need Help
 

Hello,

I purchased a 2004 G35 coupe, with 108k miles recently, and it has one mechanical problem that I can't seem to diagnose. I am leaning toward radiator issues, due to buildup inside, and possible hose issues, due to buildup inside, but additional insight into the fan assembly would help. It's not the thermostat; I recently replaced that. It's not leaking coolant, and the car is extremely clean under the hood.

When I'm driving between 80-90 MPH, and when the AC or heat or ambient are is on (heat and AC work great), the car temperature gauge begins to climb toward overheating. As soon as I back off and turn the air off, the engine cools down. I cannot stress this enough, the engine temperature climbs rapidly with the air on (15 minutes of driving on the highway with the air on will do it). Like I said, I recently replaced the thermostat and did a coolant flush. The coolant was old, nearly black, and the the owner did admit that the car had been sitting around for some time (this could cause buildup in the radiator and the radiator lines).

Is it the radiator or the line attached to it? Or is it a problem with the fan assembly? I'm leaning toward the radiator, accumulating too much gunk over time, but I would hate to make such a large investment with inadequate knowledge. Unfortunately, I recently moved and I don't have a mechanic I can trust in the area.

I was also wondering what type of aftermarket radiators people would recommend. Actioncrash, which I often use for parts, has NEW radiators from Denso, Kyo, spectra, APDI, for less than 100, but these are aluminum cores with plastic tanks. Should I pay more for full aluminum? Does it exist?

An intelligent diagnosis and a recommendation for an intelligent part to buy, for someone who wants to keep this car for another 100k miles, would be appreciated.

TY for any help, because I absolutely love this car. It drives great, shifts smooth, but this one issue is really troubling me because I like to push it on the highway a bit.

Does it still overheat on the highway with the AC off? Cooling fan failure is super common and can cause issues with overheating.

There is a chart that tells what speed and conditions are programmed to turn the fans on.
The ECM controls the cooling fan corresponding to the vehicle speed, engine coolant temperature, refrigerant temperature, and air condition ON signal. The Fan system has a 3-step control [HIGH/LOW/OFF].

See the linked image for specifics of how the fan is supposed to operate based on various conditions:

https://g35driver.com/forums/attachm...-capture-1.png