1975 ford f100. 1980 302 motor. I have no heat or defrost. The air blows, but cold. Temp sending unit shows a warm engine. I replaced the thermostat about a month ago. At the same time I blew out the line to the heater core. It was clogged. I just checked the two lines going to the heater core. One is hot the other cold. I guess it means either the core is clogged or a line is correct? How could this happen so quickly? I should blow out the line again, right? Its 3degrees right now and its my daughters truck. Any ideas? It is her first winter with the truck. It is new to her as she bought it this past october. Thoughts?

 

It could be the vacuum heater control valve is not opening.

 

it could be as simple as low on prestone..........it wont circulate if there isnt enough in there!

 

I checked that already. thanks. I will research the vacuum heater control valve now and see if I can figure out what it is.

 

The lines could be reversed if nobody was paying attention during the motor swap. It happened to me.....

 

i pull both heater hoses off the heater core and blow it out then fill the heater core with vinegar and let it soak for 20 min or so and then flush it both ways and blow it out. if i get a lot of crap out of the heater core i flush the engine and radiator as well so it dont plug up again right away. most poeple use tap water to mix antifreeze and lime will build up and plug a heater core.

 

Flow control valve is only stock on trucks with AC other wise probably a plugged heater core if only one hose is getting hot.

 

the valve will be in the heater core line itself with a vacuum hose running to it.

 

Thanks for all the help. I flushed out the heater core with a hose and now we have heat.