Nissan Repair: Heater Core, nissan 240sx, power flush

Question
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Followup To
Question -
I have a 1991 Nissan 240SX. Heater barely gets warm when engine is up to temp. Blower works. Replaced radiator and thermostat last summer. No noticeable leaks on carpet yet. If the problem is the heater core? -how do I replace it?
Thanks
Answer -
You have a bad thermostat.  Go to the dealer as it is the best thermostat you can get for your car.  However, if your dealer is like mine they will have to order the gasket.  Yes, it makes no sense that they would have the thermostat but no gasket.  I bought the thermostat at Nissan and the gasket at the local parts store.  Be sure to fill with 50/50 water and antifreeze mix and use demineralized water to keep corrosion down.  Do not power flush the engine as you stand a good chance of taking out the heater core.
>>I need help understanding that diagnosis.  What do heaters use as the heat source - is it the heated water running through the engine? Is there a separate thermostat for the heater core or are you talking about the thermostat between the radiator and engine? That thermostat just lets in cool water from the radiator cool the engine when it gets too hot. The engine temp is where it is suppose to be halfway between C and H and I can tell the thermostat does work because when I push the engine I can see it opens up and the temp drops a little. Any clarification would be appreciated.
Thanks

Answer
The heater core uses the water from the engine to heat the air in the passenger compartment.  If there is a blockage it could be the heater core.  As old as the car is there is a chance that someone used stop leak in the car and that has blocked the heater core.  To get to the heater core you will have to take the apssenger side of the dash apart and you will find the heater box.  The core is inside.