Make sure there is no air trapped in the heater core. With the radiator cap removed run the vehicle at idle until it reaches full operating temp while keeping an eye on the coolant level in the radiator and topping off as necessary. When the thermostat opens rev the engine a couple times. Short snaps work the best. Do this from the drivers seat while the heat is set to full hot, the mode is set to vent, and the blower speed is set to low or the next setting higher than low. If you feel short bursts of hot air there is restricted flow or air in the heater core. Continuing trying the short burst for about 5 minutes (a couple short bursts and than wait and then a couple more). If it doesn't stay hot its restricted.
I think the blend doors and mode doors are vacuum operated on that one. Make sure you can hear a noticeable difference in the sound of the air as you switch from full cold to full hot. If you don't than the blend door, blend door actuator, or the heat selector switch could be bad.
If you do hear a noticeable difference than I would suspect a plugged heater core. Especially if the first part above did not work and the coolant is muddy or nasty looking. You can try back flushing the heater core by removing the heater hoses from the heater core. Then using a garden hose spray water into one side of the heater core tube until clear water comes out. Then do the some thing on the other heater core tube. This may take several tries but you should start to notice an increase in heat. top off with straight coolant (not mixed 50/50 with water) to compensate for the water in the heater core.
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very good and detailed answer. There is a bleeding valve to remove trapped air from water line in engine bay just before that pipe goes in to passenger compartment.
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