Before I tear out the dash and replace the heater core - I just want to make sure there's not something more simple I should check first. Car blows luke warm and only if I have the fan on low. I checked the return hose, it's not getting as hot as the feed side. I'm a cheap MoFo so I did the "grab the hose and see if it's hot" test not a real temperature check but based on my shade-tree test it doesn't seem as hot. Best I can figure is the heater core is old and clogged up. I tried blowing it out with an air compressor and water hose, seems to help for a little while but I think that might be doing more harm than good, honestly. When I move the temperature selector I can hear the flop flop thud thud of the flap moving around. The A/C blows cold, replaced all that over the summer. It's not an Automatic Temperature Control thing, just a standard type. So - anything else I can check before I take the whole thing apart?
Could be but worth the $5 for the parts and an hour of time vs 8 hours and parts for a heater core. You getting any fogging of your windows? Every heater core I've had go bad will fog up the glass if it's leaking. If the thermostat doesn't cure the heating issue, I'd continue try to backflush the core.
Swapped the thermostat, but no difference in the heat. Blows warm/hot at low fan speeds, just gets colder the faster the fan goes.
The flap moving around is the blend door,and it would be expensive to replace. Have the cooling system flushed & refilled. There is another device there,firewall side near the core,as I recall,which can let exhaust gases into the heater,and I would check that (VERY bad design). I have done two of these heater core replacements and you have to work BOTH sides of the firewall--a real PITA. Right now,I am just a bit stumped with your car problem,I wish it was here in front of me,as I had four of these cars,and...well,parts hogs they all were. I suppose it was the right thermostat? And not upside down?
