Hi, just wanting to know if anyone can point me in the right direction. I have a 2006 Subaru Impreza wrx 2.5 turbo. 

I’m losing water from the Mishimoto and starts to overheat when The car is driven a bit hard, but if you drive the car normal it will drive forever no issues, but a mechanic has tested and said it’s not the head gasket. Thermostat and water pump have been checked and are all fine. 

Any help would be appreciated thanks. 

Depending on where the gaskets failed it might not show up on a sniff test.

 

usual place to start are check for leaks in hoses as they perish over time, check water pump etc, could also possibly rad cap.