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Hello, I am John, a Ford dealer technician.
I hope I can help solve your problem quickly and accurately.
It is very possible you have a plugged heater core. This is common on these trucks. Sometimes you can flush them but you have to do it directly. This is how to do it. Take one of the hoses loose from the heater core. Use a pair of pliers and pinch off the open hose back about 3 inches from the connector. Open the drain on the radiator. Now use a garden hose and push it onto the heater core. If it won't fit you will need to cut the hose and clamp it on. Turn the water on high. Let it run for 5 minutes or so. Repeat with the other side of the heater core. Now drain the system and refill with a 50/50 mix of green coolant and water. If it still doesn't work you will need to replace it because it is too stopped up.
I have never seen a v8 f150 have a problem of low coolant flow so I don't believe this is your problem. The only other problem I have seen is where the temperature blend door itself is cracked and causes lack of heat problems. The difference is this won't even get hot for a few minutes. This leads me to believe that you are dead on you diagnosis.
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I got you on the low flow ;) Sometimes the tabs break on the connectors. You just have to replace the tabs. You just squeeze the tabs, twist and pull and they come off. Not always that easy but as I said sometimes they break. You can get new ones at a dealer for around $5 a connector. Easy to install. Never heard of the ss blend door, that sounds cool!
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