2008 GP heater only blows lukewarm air

[Billski]

2008 GP with standard basic HVAC system (not auto temp, not dual) suddenly started blowing only lukewarm air when heat set to full. Engine temp in normal range, thermostat operating normally, heater hoses both feel hot. Radiator checked cold, completely full, no signs of Dexcool sludge at filler neck. Cooling system has not been opened for any service/repairs.

Checked the blend door actuator, it responds to every setting. When temp control turned to full hot setting, can hear the blend door closing against the seal and the positioning notch on the actuator shaft points straight down (I believe that's the index mark for full hot).

Everything else about the HVAC system works fine. Fan runs at all setttings, heat/dash/defrost vents all function normally, interior air recirculator functioning normally.

Other than back flushing heater core, followed with flushing/changing/bleeding coolant system, any ideas?

[Tech II]

If both heater core hoses are hot, then there is no need to backflush the core.....just because the actuator is moving doesn't necessarily mean the temp door is.....the temp door linkage could be broken.....only way to know for sure is remove the actuator(2-5.5mm screws), then manually turn the temp door and see if it actually moves and the temp changes (blower has to be on).....

[Billski]

Thanks Tech II. I pulled the actuator and manually rotated blend door to full hot and full cold. No signs of drag, can hear the door closing both ways. Held blend door to hot manually and indexed the white blend door shaft with a red Sharpie against a rib on the housing. Replaced actuator, it turned to full hot and index mark aligned perfectly.

Problem still exists. It will blow hot at first, then progressively loses heat and intermittently gets hotter then cools down to lukewarm. Watching the temp gauge on the 25 min drive to work (part city/mostly highway) and I do believe I'm seeing the thermostat sticking open or at least partially open. On my next decent day off, I plan to replace thermo and do a coolant flush/ change. At 77k miles and almost five years, it's time for the DEXCOOL to be changed anyways.

I'll post back to confirm the fix or seek more help.

[Billski]

Replaced thermostat - that was the issue. It must have been sticking open after initially getting up to temp. Now have "burn the feet heat" all the time.