Hi new to this forum and my first issue... So I have a 2006 Sonata 4cyl and the heat has been slowly dwindling down to nothing and with the negative degree days its been very cold in the car. I just changed the blend door actuator, however it did not solve the problem. When slowly moving the temperature control from cold to hot you can hear the door open and close and when heat is on full blast it comes out lukewarm at best. All of the radiator fluid levels are right and the car runs at normal operating temperature. Any ideas on what it could be? Is there something I'm missing I just can't figure this one out and am trying to avoid costly shop fees. Thanks in advance for any responses.

 

You heater core could be clogged with debris which slows the water
Or, the heater core has "stuff" on it which slows the transfer of heat.
You can try reverse flush of the heater core to see if it will clear it out??

Check any valves leading to the heater core in the engine bay
See if they move to allow hot coolant flow to and from the heater core??

Good luck

 

Thanks for the quick response. I will be checking the lines today when it is hot from running. Will update post when I find out more.

 

Maybe, some addl info may help shine some light on the problem.
If you checked the hoses to and from the heater after the engine runs a while and normal operating temp is reached , if you found no valves in those lines both hoses should be about the same temp if the heater core is not restricted as "avisitor" suggested. If not , the core is restricted or a flow control valve is not fully open. If both hoses are hot that would mean coolant is getting in and out of the heater core.. What could be blocking full heat again as a visitor suggested could be dirt on the heater core preventing the transfer of heat. If you have a blend door , that may seem to be working , but may not actually be moving internally. That would need to be confirmed operating correctly.
If better heat is produced when engine RPM is increased it would indicate there is air in the heater core preventing full circulation.You can allow the trapped air to exit by loosening and removing the higher position heater hose just enough to allow coolant to push out the trapped air. This is best done when the coolant is not too hot where any coolant could get out and cause a burn. Let us know what you find, thanks.