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This issue normally indicates an air pocket trapped in the heater core. When you are going down the road the water pump forces the air out of the heater core and it can get the coolant in there to generate heat. I would start by checking the coolant level in the radiator and if it's low top it off. If it doesn't look low on the cold engine then the system will have to be bled to get the air out. If there has been any work done on the cooling system then it needs to go back as maybe it wasn't bled properly.
Also, sometimes having a flush and fill done with a machine will remove all the air on stubborn systems.
Which engine do you have in the truck?
Ok, sorry for the delay, I wanted to get you a copy of the bleeding procedure.
HERE is a link that should get the air out of the heater core for you.
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Ok, if there is a leak you will get air replacing the coolant and it will end up trapped in the heater core.
I am not saying you still have a leak, but that could be where the air pocket came from. If you get heat, then lose it again, have the cooling system pressure tested.
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Ok, then this should work to get you heat. If not, check and make sure the heater hoses into the firewall are getting hot and we can go from there.
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