Heater problems

I am having problems with my heater, I can start my car and turn on the heater and it will get warm inside but not hot. I can drive around in town and it will stay pretty comfortable, but if I go out on the open road then it will blow only cold air. I have replaced the thermostat and flushed the radiator system. When I turn my heater on I have to flip my venting to the ouside air to even pull any kind of heat, if I vent it through the inside venting it just stays cold. I have to put it on the defroster and the air stays warm for a bit and I can feel hot air coming from my vents. My blower motor works just fine. I was wondering if anybody had any information on what possibly might be the problem, a defective heater core maybe? I have a 1997 Rodeo, 3.2L 4WD V6. If anybody can help me out that would be great. Thanks

double check cooolant level
then check blend door operation

In addition to the things 2Eye mentioned, I would do this-

Warm up the engine completely and set your heater controls to hot, vent air to dash / floor and blower on high (4).

Check the two hoses going to your heater core. They are located on the passenger's side between the engine block and firewall.

One of them should be hot and the other should be hot / warm to the touch.

If they are cool or cold, I would suspect a clogged heater core. Sometimes the heater core can be back flushed and cleared out.

If they check out OK, the blend door is probably not working properly.


Good luck,
Dale