HI, Replaced heater core last winter. Worked well but now it’s heating slowly and weakly. Replaced water pump – heater still not really working well. Mechanic cleaned (blew?) rust out of something, but is out of ideas. Any thoughts? Thanks!
Are the engine and the heater core reaching normal operating temperature? If not the thermostat may be bad.
If the heater core is getting hot (feel the hoses), the the problem is with the blend door inside the HVAC system. The blend door controls air temperature by determining whether or not the air flows through the heater core. If the core is hot but the air is not going through it, you won’t get much heat.