I have had my pre-owned 2012 Mazda CX5 2.2 diesel Turbo out of Japan for about three years now. From the time I got it I have had to change the head gasket and trouble shooted just about everything that could go wrong and changed a myriad of parts. My issue over the past six months is coolant being forced from the radiator back into the overflow/reservoir.
I Initially noticed an issue when the vehicle lost power one night and the engine temperature warming came on. I changed the radiator cap and put in coolant and had the mechanic check it for leaks of the hoses etc and it did this overheating maybe once more.
However I have had to be transferring coolant from the reservoir back into the radiator every other day or right after f I do a long drive. It doesn't appear to be leaking as a leak down and compression test was done and we note that the coolant piped back into the radiator fills it right back up. The oil level is good (it had just had an oil change before this' issue (as it had been parked for another issue for months prior).
The mechanic wants to put stop leak in the radiator, others say bad idea if there isn't a leak.
I am out of ideas and can tell that the vehicle isn't driving as good as it usually does although there are no warnings being displayed.
hELP!!!!

 

Welcome to the forum

 

Sounds very much like the head gasket is leaking again. Compression gasses are entering the cooling system and pushing the coolant out of the radiator and into the overflow reservoir.

 

Thank you