The combustion pressure is always higher than the coolant pressure. So when they leak you pump combustion gases into the coolant when the engine is running. Causes air pockets. overheating, loss of heat, constant bubbling in the coolant tank. May even see soot in the coolant bottle. When they really are bad they will leak coolant into the cylinders when the engine is off. May see white smoke and rough running at start up can hydro lock the motor bend rods ect.