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Possibly the thermostat sticking open. How big is your radiator? I once had a 77 with a 4 or 5 core rad that wouldn't throw heat til I put a winter front on the truck.
Not correct ratio of water to antifreeze maybe? I had a buddy change the fluid in his Ranger, put too much water in and it froze. Only noticed it was frozen because he wasn't getting any heat blowing in the cab I guess.
How bout a trapped air bubble?
imo our trucks really dont get bubbles i never had one in our trucks. my 79 will burn you out of the cab my 91 v3500 crew is ok as long as its not too cold or it does the same thing and wont warm up. one option is to block part of the radiator off. i wouldnt jump head first and block too much at first and i would use cardboard