Heater problem

The inlet heater hose should be hot. The outlet hose should be cooler.

If both are the same temp or close to each other. The core is plugged.

The heater core works in the same manner as the radiator.

 

Plugged core.

 

Probably should not share this but I was a teenager. 1958 Plymouth Savoy with that problem. The radiator had been leaking and I put stop leak in it. Radiator leak stopped but so did the heat. Went to the do it yourself car wash. Disconnected heater hoses from engine and used the pressure washer in hoses to core back and forth. Cured: I was a 17 year old genius and never had a problem with it after that.
I would not try that unless you are at home. Cores today are likely not so strong.
A good reason not to use stop leak.

 

Did you replace hoses by chance? I got mine backwards on the bunk once. Had to swap then around on heater core to work. I don’t know about yours, but much to my surprise, mine supply’s cab heater core from block, returns to Waterpump. Always thought it circulated opposite. From Waterpump to heater core and returning to block. Seemed to make sense. But I was wrong.

 

I worked on a Sterling that had one of the duct doors broken off inside. Could not see it from the outside. It was a one piece plastic flap with a stub on each side. Maybe check those out. Mine was four hours plus of fun. Most all the dash out of it.