08 2.4l Aura
A while back, I noticed my temp gauge was inconsistent; after a few days, the car started overheating. Along with overheating, there was no hot AC. At first, I thought it was the water pump because it was leaking from the weep hole, but since replacing the pump, there is still no success. I have replaced the water pump, thermostat, and coolant temp sensor. I ran a hose through the heater core, the radiator, and the upper rad hose to the water pump to make sure nothing was clogged. I thought that it was an air bubble in the system, so I ran a vacuum with an air compressor like you would to do a coolant fill, and the vacuum would slowly release. I vacuum filled it with water anyway, and it slowly loses water without ever running it once. As of now, my coolant reservoir is empty. While running the car, there is no evidence of the water mixing with the oil as I have checked the dipstick and oil cap and even drained some oil from the pan to see if there was any water at the bottom of the pan, and there wasn't. At this point, I don't see where to go from here.

 

One of my L series with a 2.2 would slowly loose coolant over time and it was a heater hose fitting at the heater core. I had to pressurize the system and watch closely to find the very slow leak. It had factory quick connect fittings and one had a slight crack near the o-ring.

 

The best way to check for coolant leaks is with a cooling system test kit. A hand held air pump with pressure gage, several radiator and surge tank caps to fit most vehicles, allows connection to the pump. Pumping up pressure to the cooling systems rated pressure on a cold engine allows searching for coolant leaks without working on a hot engine/hot coolant. Autozone have free loaner tools.