coolant issue

[Lio]

Hello,
I did some search on this forum but did not find the exact same issue that I am facing.
I have a turbo D / JX engine from 1987, and that has been through a major rebuild.
Now the issue is that when the engine is getting hot (after let's say 20 miles / 30 km) the main coolant tank (the one inside the engine bay) is getting empty while the one behind the plate is filling up. I have put a new "blue" cap one the main tank but still have the issue. it's seems also that some coolant disappear along the process but could not find any leaks.
I have bled the system several times via the the screw at the front and top it up via the main tank, still have the issue. 
One more thing : when the engine is warm and the main tank is empty (leading to the blinking light in the instrument cluster), if i open the blue tank carefully, the coolant is coming back from the engine.
what do you think ? what should look for ? thanks for the support 

 

[Stesaw]

Sounds to me like you got exhaust gasses getting into the cooling system and its pushing coolant out.

Something not right with the rebuild by sounds of it.. maybe the wrong head gasket or the heads not torqued down right...

[mrhutch]

“Sniff” test on the coolant needed.

If positive it’s time to whip the head off again.

[mioba]

JX are well known for the heads to crack between the valves.Engine gets hot, crack widens and exhaust gas in the coolant.

been covered to death in years gone by.

[multisi]

Were the head bolts re torqued after the engine rebuild ?

[Lio]

Sorry for my late answer...
I have been busy with some other jobs in the house..but now I am back on the T3 and decided to go for a new head gasket as I am not sure how good it is right now . I will check the head for cracks as well.
I started the job tonight and got puzzled by the poor condition of the timing belt that is actually less than 500 km....I suspect some kind of misalignment..the injection pump is new or reman.