Hi
I am new to the forum...I own a 535 ia '92 model

This week my mechanic (not a BMW specialist) found a crack in the hose between the thermostat and the water pump that was the reason of the leakage of coolant from the system. He replaced it and used water to replace the coolant missing, around 2 gallons. This was done 2 days ago.
I haven't used the car to much, but anyway this morning after coming from an errand and after parking the car at home I noticed a hissing noise that after opening the motor cover found out that came from the coolant reservoir where the water/ coolant was boiling. I used almost 1 more gallon of water to come to the level indicator on the coolant reservoir; this done after the system was cool.
I haven't talked with the mechanic yet but most probably my understanding is that this is the result of using water only instead of the mix 50% water/coolant...anyway will gladly accept any suggestion on how to fix the problem...i.e change the thermostat, or other if necessary…

Tks

Note: the gauge indicator in the front panel never went over half mark where it is normally set.