Flush or Replace Heater Core

With the engine at full operating temp, locate the 2 heater hoses where they go into

the firewall and feel them to see if they are both hot to the touch. If not too hot to

hold, then you have to look at the actual engine temp and if that is up, if it is, then

your looking at a restricted heater core.
If you find that they are both too hot to hold, then the heat in that vehicle is

controlled by a blend door that regulates heated air flow. It is operated by an

electric motor/actuator. The problem can be that the actuator is stripped or

inoperative or the door itself could be damaged. This is what needs to be

determined by examining the actuator and see if it is responding to heat change

commands or not.
If they are both not too hot to hold on to, then you either have a restricted heater

core or the system is not completely full and getting air pockets for some reason.