I am working on a 1980 mercury zephyr there is no coolant cirulating throughout the heater core. I have changed the water pump, which didn't fix the problem. Can anyone tell me what to do to fix th

like someone stuffed the coolant jacket with steel wool, then allowed it to rust, by giving it a Wisconsin Salt spray experience? ....no kiddin' aside would ask is there flow or chipmonks found a place to store their stash for the winter?....obstructed entirely or could be easily blasted clean with a charge of compressed air...is what I'm after?

a shop-vac might pull a few avocados out of there too~

Would air in heater core cause this U think instead of blockage. I have blown air through it.

33 year old heater core? Or have you replaced it? If it's an old core and car has been sitting for a while it might be stopped up. Try flushing and back flushing the heater core with a water hose. If a lot trash comes out I would advice changing it. If the water flows out freely then inspect for collapsed hoses ect. Have seen older radiator and heater hoses fall apart internally blocking the flow of coolant thru them.

How do you dack flush ..

Remove hoses from heater core then stick water hose on one of the heater core pipes and flush water thru it. Then swap water hose to the other pipe on the heater core and flush water thru it that way.

typo....man...Duck flush is messy~

we used to have a fitting...more of a pipe with a Schroeder valve installed in it that we could blast a bubble stream to "scrub" the corrosion away....of course this is completely environmentally NOT responsible as birds drink the ethylene because it is sweet....then die~

Oh and also inspect the heater core hoses for an in line valve. Not sure about your model but some have a valve in the hose that has to open to let the coolant flow thru the heater core.

Back flush I meant