Heater core ?
 

I replaced my heater core a couple of weeks ago and it doesn't seem to heat half as well as it did. What all is related to the heater ? I recently replaced the water pump and thermostat, timing chain and a few other things and did a coolant flush late last Summer. The blower motor seems to work OK as well but am not sure about the heater resistor. What purpose does it do as fas as the heating goes ?

Thanks again,

Did you check the positioning of the doors in the heater core plenum? That might be where the issue lies, especially if you're sure that the cooling system is correct and proper.

The resistor assembly controls blower speed. Each speed (except for high) has a resistor of a different value associated with it. HIGH has no resistor and the blower runs full tilt.

I would be more inclined to pursue g_k50's suggestion and check for vacuum control pots under the dash that may have hoses come loose or one of the air vanes attached to the control pots stuck or a linkage disconnected.

Are you saying the heater motor blows plenty of air, but the air coming out is cooler than it used to be? If so, you may have simply replaced the thermostat with a cooler one than it had previously. Is the temp gauge showing the engine is running cooler than it used to?