My 1992 Loyale heater worked fine until I changed the engine. I assume there is air somewhere in the system but disconnecting the heater hoses and blowing water through the heater core does not help. I have tried squeezing the upper radiator hose with the nose pointed up. No help. Replacing the thermostat. No help. I have tried running the engine with the radiator cap off. Still nothing. Any suggestions? What else can I do?

Perhaps there was debris in the cooling system of the replacement engine.... the cores aren't too hard to clog up.

 

Other than that, make sure there is flow through the heater core lines to the engine.

 

You may have to pull the dash and replace the core. It's fun, you'll like it

 

GD