The engine doesnt overheat because of the water pump at all the water pump leaks because it is the weakest point in the coolant system,which becomes overpressurized due to head gasket failure due to carbon stamping,This is due to the burning of oil via the pistons-and rings ,the egr becomes clogged because of this excessive oil burning and deposits.Toyota have been great with extending warrantys and paying in full for parts and labour in most cases.