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My 05 MDX has developed an odd problem - the heat has become lukewarm. Both the front and rear heat blows the same lukewarm air when heat is turned up to max. I've done everything I can think of to diagnose but I'm stumped. The following is a list of what I've done so far:

Replaced antifreeze (Honda Type 2 Coolant).
Flushed heater core (Flushed using boiling water and detergent. Water flowed freely in both directions an no dirt or debris came out).
Verified the hoses going into and out of the heater core are warm.
Verified blend door actuator is operating properly. (removed blend door actuator and disassembled the gears. I noted no worn gears so I reassembled with plenty of grease and reinstalled. Verified it is fully operational).
Verified no antifreeze leaks.

I replaced the thermostat and water pump six months ago with honda oem units when I did the timing belt. I believe the thermostat is working properly. When the engine is cold only the top radiator hose is warm while the bottom one that goes to the thermostat housing is bone cold. It will stay this way until the thermostat opens and then the hose will warm up significantly. Note - The air blows lukewarm both before AND after the thermostat opens.

To summarize, I have brand new anti freeze, new water pump, new thermostat, flushed heater core, verified blend door actuator is working (as well as the blend door itself), and verified no antifreeze leaks. I also bled the system for a solid 30 minutes using one of those radiator funnels (like this - https://www.amazon.com/OEMTOOLS-87009-No-Spill-Coolant-Filling/dp/B01A2CQSU6/ref=sr_1_5?keywords=coolant+funnel&qid=1573835200&sr=8-5) so I do not believe air is in the system.

Am I missing something????
 

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Check that the heater valve (on the passenger's side firewall) is being fully opened. On mine, the actuator wasn't pushing quite all the way. There's room to reach down and manually push the valve into "full hot" position - have someone cycle the temperature control (engine running) from full cold to full hot to see how much the arm on the valve moves, and then see if there's more room to move it. If there is, it's just a matter of readjusting the cable (plenty of online info on that out there, as I recall).
 

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Easiest place to start is where habby mentioned. If that doesn't seal the deal on the problem, try running the engine from cold with the radiator cap off. Once hot, if you do have hot heat, could be a head gasket issue. Running the engine with the cap off lets air migrate back to the radiator neck and out, hence killing air locks in the heater core, which is basically a garbage collector for the entire cooling system. On an ultra-rare side note, if any 'Stop-Leak' was ever added to the cooling system...guess who picks up that garbage (just like air)...Mr. heater core. Just because you flushed the core and saw good news come out, doesn't mean the entire heater core is being exposed to hot coolant. Again, this is very rare but always possible. Let us know how it turns out.
 
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