Replaced heater core, now only lukewarm air?
 

1994 Volvo 940 sedan
Non-Turbo
Automatic
150k miles

So, the heater core had been slowly leaking and it was time to replace it. Bought the heater core, replaced it over the weekend (which was pretty difficult). After replacing it, the system did not take all the coolant back in, after taking off the hose at the highest point all of the fluid finally went back into the system. I'm assuming it was bubbles in the system. Anyway, now that everything is all back together, when I drive the car the air only is lukewarm that comes out. Not cold, not hot, but pretty warm. I felt both hoses going into and out of the heater core at the fire wall and they are both hot. All hoses are hot actually. I cannot find out why it is not hot at this point. Could there still be bubbles in the system causing that? There was stop leak in the system before, but it was strained out prior to the install of the new heater core, so I doubt that is an issue. Either way, any insight and tips would be great!

Thanks all

Probably still air in the system.

That's my guess too. I have it up on ramps now and will see if I can burp it some... Any other tips in the meantime still welcome!

make sure the heater valve is opening fully, and that its not obstructed

Would both lines in and out of the firewall be hot if it were obstructed? IPD only a couple miles from me so I suppose I could head over and grab one, but I bet you know the answer to that question...

Yeah, right...
You have to check the possible air in the system and the valve function--there is nothing else! The hoses getting hot is a good sign but it all depends on how fast it takes for them to heat and how much flow is passing through. Also, this time of year it may take a while for the heater to get up to operating temp.-- especially with a "hot" thermostat (one that takes longer to open).

Make sure you have all the vacuum lines reconnected properly, as these are the controls for the defroster, foot heat and more importantly the temp settings on the units without autoheat control.