Pontiac Repair: 98 transport 3.4, question pool, heater core

Question
I just had the van in the shop and the mechanic on hand stated the 3.4 litre automatic do not put out much heat.  The problem is to -20 lots of heat but belowe -21 there is hardly any heat at all, mostly luke warm.  I've checked the heater core, works fine, thermostat was changed, not part of the problem I realize, is there a louvre or something that could be stuck partly open?  We do not move the temp at all so I'm lost.  Any help would be greatly appreciated.

Answer
sorry for the delay this question was in the "question pool" and I had to fish it out of the deep end. It was not addressed to me. You could have a partially plugged heater core in this when the engine is up to temp see if there is a difference in temp between the inlet and out let hoses to the core. If they are the same pretty you more than likely have an obstruction in the core take the hoses off and back flush the core with a garden hose. You could also have a door issue inside the HVAC assembly the doors on this van are electrically operated and it could have a stuck/binding door or a actuator for the door not working properly. One other thing is does the engine cooling fan run all the time? if that fan is onn all the time when the HVAC is in heat mode that could be overcooling the radaitior and causing the low heat issue. just a few ideas...