Heater core replacement mistery
 

'93 A6
After replacing the leaking heating core some things are happening now:
The heating air gets never warm.
The engine temperature is below the normal (stays about at 80C).
Some coolant loss , small but real.
If AC is 'on' the the gauge temperature drops immediately.
Wondering how to connect the current events each to other and what could happened during the core change?