The car had in the beginning had water leakage , inspection of the oil showed the oil is mixed with water, the mechanic concluded that the head gasket need to be changed. The job was done. Before testing the car was filled with coolant water , surprisingly and before starting the engine the coolant was leaking and not externally, but it was internally as the coolant was seeping from the drain plug when open,
The spark plugs were checked, the combustion chambers too , they were ok.
I hope this explain the situation for you to find out what went wrong.