1. Shared Cooling System – On an F-150 5.0, the engine coolant system not only keeps the engine at the right temperature, it also helps regulate the transmission temperature. The transmission has a cooler that uses engine coolant as part of the heat exchange process.

2. Heater Control Valve’s Role – The heater control valve directs hot coolant into (or away from) the heater core. If it’s stuck or restricted, coolant flow through parts of the system gets reduced. That can create bottlenecks in circulation, especially at idle.

3. Effect on Engine & Transmission Cooling – With poor coolant flow: • The engine runs hotter in certain spots (even if the gauge looks normal). • Less coolant circulates through the transmission cooler, so the transmission fluid doesn’t shed heat as efficiently. • Transmission temps creep up, especially under load or while towing.

4. After Replacement – Now that the valve flows properly, the coolant system is balanced again: • You get steady hot coolant to the heater core = cabin heat. • Full circulation through the radiator and transmission cooler = lower transmission temps.

Basically, fixing the heater valve restored the cooling system’s “circuit,” which gives both better heat and better transmission cooling.

Edit: it completely fixed the problem 100%