So I just got back stateside a couple days ago and on my way home from a new years party noticed that the heater decides when it wants to work. Cranked full blast it'll blow really hot and then go limp and not feel much warmer than the 19° outside, then it'll get warm again for a bit. There is no rhyme or reason that I noticed. Engine is up to operating temperature, coolant is full, coolant filter was replaced 1500 miles ago (not that it would make a difference). Any thoughts?

 

Check if the heater core is partially plugged up, force some water through

 

Could be the blend door actuator.

 

Does it get hotter under acceleration? Could be air in the coolant system. Its important whenever filling the rad to turn the defrost on full blast to help avoid air bubbles trapped in the cooling system

 

I did a drain and fill (not a full flush) and replaced the coolant filter just for good measure, heat is back with a vengeance now, even just idling to warm up in the driveway for a few minutes with the heater on she is nice and toasty inside now. Maybe have been an air pocket that got burped out during the fill.