The heater in my 97 B2300 quit working. The thermostat seems to be functioning because the engine heats up to normal temp and acts normal. I've had the radiator flushed and filled, and I replaced the radiator cap. When I crank up the heater temp **** I only get a luke-warm airflow. The blower works fine. There is no leaking at the heater core, or anywhere else. There doesn't seem to be any loss of power, like from a blown head gasket. I'm wondering if it could be something simple like the **** linkage. Any ideas?

 

If the heater connections are both hot after 15 minutes of driving, then suspect that the air isn't being diverted over the heater core, or your model has a valve that opens/closes flow to the heater core.

 

Mine is like t hat, but I'm pretty sure it's because the ducts have come loose. Check to see if you turn it to defroster that you can also feel it coming out the other vents, good sign your ducts have come loose under your dash. I noticed that with mine and felt the vents and when you have it turned on you can feel a bit of air out of the other vents, and if it's not connected that would explain why the air isn't all that warm, before it gets to the vent it mixes with the cold air.