If the heater core was flushed it should be fine. As far as determining if the rad is plugged, try the following.
With the engine COLD, remove the rad cap and start the engine and allow it to warm up. When you hit normal operating temp, the coolant should start to circulate in the radiator. If it doesn't chances are the radiator is clogged.
As far as the smell and leak, it should have no affect on the heat. As long as the coolant level is full, you should get heat.
Also, if your heater core is leaking, you would most likely see it in the vehicle on the passanger side. Also, most times the coolant / steam from the core leak will fog the windshield. And yes, to replace the core you need to remove a good part of the dash.
I just reviewed you posting about checking the heater core hoses. If one was very hot and the other only warm, it sounds like the core is plugged.
Another thing could be the blend door which is located in the HVAC ductwork on the passanger side under the dash. If it isn't opening, the air doesn't pass across the heater core to provide heat.
Let me know if this helps or if you have other questions.
Friday, January 2nd, 2009 AT 12:52 PM