Engine cooling system coolant flow direction diagram

Coolant Flow for 4.6L modular engine

Note that: before the t-stat opens (engine cold), ALL of the water pump's pressure goes thru the heater core. So the high coolant flow at that time will self-bleed any air out of the heater core, but it can also overpressure the core, which is why Ford is now adding a restrictor to many vehicles. See the TSB in this caption:



Smallblock V8

Owner caption January 2005 · Photo 73 of 980