head gasket concern

hey folks
i have a 2009 m13a. engine was replaced december last year due to head gasket leak - confirmed co2 color change test.
everything was great the past six months and then she started puking large volumes of coolant to the expansion tank.
did like 10-15 co2 tests and all negative - radiotor and expansion tank.
no over heating, replaced cap, replaced upper rad hose, lower looks fine, heater works fine, thermostat is original maybe a year old, no visible leaks, no coolant smell in engine bay and no oil in coolant. when bleeding no constant bubbles (sporadic ones but it was the same on the last engine so sensitivity of this test is questionable).
when expanding to the tank and followed by cooling it does manages to suck some fluid back and upper hose gets softish.
tired of going back and forth with the GPT, screw that - someone has any idea what else is going on?

Does it suck all the displaced coolant back? If it does, I'd look at cleaning the radiator inside & outside, then the thermostat.

If it doesn't, there is A Problem.  Wait until it is completely cold and take the radiator cap off, it should be full right up to the tippy top with coolant. If there is a space in the top likely head gasket, but you say the hoses are soft, so I'd expect it to be fine.

While the engine is completely cold one thing you can check without messing with water is the fan coupling, it should turn with a bit of resistance when cold. If it is free to spin or rattles the viscous coupling is gone. 

 

cheers mate.
fan feels good, no free spin or rattling. no over heating as well. overheating comes when most of what supposed to be in the rad finds its self in the expansion tank.
probably will change the engine, may Christ be on my side

You said it wasn't overheating!

I'd look at the radiator cap.
If the pressure seal (or the ring in the radiator) is damaged or the spring is weak (I don't see why, but you never know...) then it will blow most of the water out into the expansion bottle as it starts to boil, but should draw it all back as it cools. (unless the expansion bottle over flowed, then the overflow will be lost)

 

not over heating in general, it gets warm when its not sucking enough coolant when cooling down, over time no coolant left in the rad, havent got to the heating thanks to my friendly OBD so i stay on top of it.
i bought *2* new original caps, looks he's not the first suspect.

If it's gradually pushing the coolant out to the expansion reservoir and not drawing it back, and by that I mean, over a period of several drive cycles, I would take a close look at the expansion tank "plumbing" - if the lines from the radiator filler neck to the bottom of the expansion tank pickup tube are not "air tight", the system cannot pull the coolant back as it cools down.