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Location: Sassamansville, Pa
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The inline valve is not a back flow control, but a valve that closes off coolant circulation to the heater core to get the cab cooler when using the A/C.
There should be a gray vacuum hose that connects to it.
You could also get the truck up to temp and feel the heater hoses, the hose with the valve is the inlet hose and the other the outlet. See if they feel the same heat temp. If there is a noticeable diffrence, you may have a bad inline valve that isn't opening, air pocket, or blocker heater core.
If both hoses feel the same in temp, you may have a blend door that is not working properly.
Edit: is the coolant in the degas bottle at the proper level?
2004 F350 CC DRW King Ranch
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