Head gasket could be bad
Probably a blown head gasket. Get a technician to look at it. If it is, it will be a major ordeal.
The headgasket has failed.
It could be caused by a defective pressure cap on the radiator cooling system.
Radiator cap could be lose or wrong size
there might be a leak some where in a coolant line. or maybe you have a leak in your radiator.
Defective water pump.
Possibly head gasket.
because you probaly changed it with the wrong water
heatercore leaking! also change thermastat.
What joint exactly is leaking on you p-trap?
Probably a blown head gasket. If the car has an (engine) oil cooler it might be a leak there too.