Quote:
Originally Posted by spa
unless we drive at those high RPM's
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Is it RPM or SPEED related? I don't see how it could be related to rpm but speed does make sense because if you don't have the system set to recirculate the air then the faster you drive the more pressure the incoming air will use to push into your system. At any rate if your heater core is getting hot then your problem starts with the HVAC system. You need to dig into the HVAC Section of the service manual found on this site and start troubleshooting. A flap somewhere isn't opening/closing or your blower isn't operating correctly or you could have a physical clog in the vent system (like a rag, birdsnest, etc no matter how unlikely). You didn't say if you are receiving proper airflow at the vents. Can you feel a noticeable change when moving the fan speed between the lowest and highest? Do you have the same symptoms with the Air Conditioning, does it also not get cold enough?
If your AC works properly and you are getting the correct airflow for a given speed setting out of your vents then you most likely have an obstruction at/near the heater core preventing proper airflow across it.
Find that section of your manual and post more information if you want more help.