I have a 91 E350 with the 7.3 diesel. The heater core blew and I have been trying to replace it. I've got the new one in but I am stuck now as I did not map out where everything went when I took it out. Does it matter what I hook up to the top or bottom port on the heater core? Also, there was a T fitting three way of heater hoses. One was 3/8 and the other were 5'8's. But it seems that the ports on the heater core are bigger than 5/8ths. Am I missing something here? Anybody have a picture of what goes where?
The hose ID should be 5/8" which would be 3/4" OD. If your '91 is like my '97 both hoses would be the same ID/OD
The heater core is kinda "dumb" in that it doesn't really care which tube you use as an inlet or outlet---it just holds hot coolant for the blower to remove the heat and push it into the cabin. Typically the top would be the inlet, receiving the hottest coolant first, from the port nearest the thermostat housing.
IF you're reusing the factory formed hoses there might be a restriction purposely placed inside one of them, reason unknown but I'm guessing its to slow the coolant flow in order to extract more heat. IF running your own new hoses this might not be that important.
Not sure about the diesel three way hose connection so maybe you'll want to track down where the smallest of them ran?
Sorry I can't be of more help but a photo of your own current situation might be helpful to someone here.