Agreed. That is the only thing that makes sense. It's a messy job removing the heater core, and it's too cold for that right now. I'm hoping that it warms up in a week or two. Thanks

 

Are you sure the blend door is working properly?

Another easy test is to feel both hoses to the heater core when the engine is hot. I'd recommend using gloves. Are both hoses about the same temperature?

 

That's what I was suggesting but the OP has concluded the issue is the heater core. I honestly think the heater core being the issue is extreme but I'm not there. I'm sure the OP knows what they are doing and hope the issue gets resolved. Heater core being an issue in these models is not abnormal.

 

I agree that it's unlikely, but it seems like the issue is either the heater core or something blocking the heater control valve. I will check to see if both hoses going to the valve are hot, and dig into the problem more on the next warm day. Thanks again.

 

If something is blocking the valve or the heater core, the same result is that little or no coolant circulates through the hoses. The return hose will be cooler.

From the engine compartment, you MIGHT(?) be able to disconnect hoses & try to flush through the heater core with a garden hose.

 

My thoughts, exactly. I'll start with that, and only pull the heater core if I can't clear the blockage with a garden hose

 

I'm not sure which is the inlet vs. outlet hose, but I'd start with flowing backwards through the core. But try both directions before giving up.

 

Kind of related question, but what if you can get water to flow thru both hoses (both directions), would that mean a damper door? Or the fins on the outside of the core are blocked? Just asking.

 

I was really thinking that flowing one way or the other would flush the crud out of the core, fixing the problem.

I guess it didn't occur to me what if it flows nice & free right from the beginning without any crud coming out of the core. In that case I'd look at the other side of the hoses. Wherever they are disconnected, if it flows nice through the heater core, then I guess the obstruction is in the hoses (& valve) connecting back towards the engine.

How do you show the damper doors are actually moving correctly? What if the external levers are moving but inside the heater box the doors themselves aren't?

 

Thanks Jim. My son was having a heating (or lack of it) issue with his Civic is why I was asking. In his case water flows both ways freely, and has new hoses and a valve in place, but is still blowing cold air. It's also not the AC either, as the pump belt ad been cut after the compressor seized. WE're both thinking damper motors, but maybe something blocking the heater core too. Maybe even a mixer box if they use one. He does have an FSM now, and I had suggested he go thru that section to see what Honda says.