Any idea on what may be causing a coolant leak in my 2011 g37XS? engine temp was higher than normal and upon checking it was out of coolant. I have filled the overflow reservoir a few times and have bled the air from the system every time. However after driving for about 30 mins it empties everytime. Did an oil change to rule out the head gasket and it was clean. Also see no visible leaks on the ground and the engine cover is off. Any suggestions?

 

If you are losing coolant at the rate you describe then you have a substantial leak somewhere. Look harder. Check:
- heater hose connection;
- all hoses/ connections (radiator, heater, throttle body);
- drips near the AC compressor; and
- unlikely, but check the passenger cabin (in the event the heater core sprung a leak)

A slight leak would not "empty" the system in 30min. or less. A leak of this scale would definitely be noticeable. If need be, refill the system and have someone rev the engine (2000rpm MAX) while you carefully inspect under the hood and under the car.

 

My guess would be the heater hose fitting is shot and leaking all the coolant you put in out. The EX35 we had, it had a small leak from the heater hose fitting and it would slowly cause the engine to overheat, overflow tank would be empty as well.

 

If you are interested in fixing it yourself you will have to purchase a radiator leak pump.

It's a pump that goes on your radiator and pumps air pressure into the system. Most coolant systems function between 15- 20psi which will be what the pump will do to your system.
Differences are the only way on your own to create this would be to turn the vehicle on which heats up the engine bay and lets the leak dry before it reaches the ground.
Pumping while cold will yield the leak.

If you dont feel comfortable with your skills then take it into a shop and let them find it.

Like @Krzysztof47 mentioned. Common for the heater pipe connection to disintegrate and cause a leak. These leaks wont be noticeable until it builds the correct pressure and shoots out similar to sprayed fuel. It will evaporate before you notice it.

Good luck.