Hey folks question for you. I have a 1998 durango I started it this morning -5F brrr. But the heat only would be luke warm at best, when I started to drive warmer air came out then when I would stop it cooled right back down. The engine heat showed it was up to normal temp. Any ideas what the problem might be. It has been working fine until this morning???

 

Hello and welcome!
If your fan is still running then more than likely your heater core is a gunked up. What happens is grime builds up on the veins (its just like your coolant radiator but smaller) and somewhat insulates the veins preventing the heat transfer to the fins which radiate the heat into your cabin. You could always try getting the core flushed, but if the problem persists you may need to get your heater core replaced (Could be costly).

 

Check the coolant level. If it's too low there may not be enough to get to the heater. In winter weather the cold air will help cool the engine so that you may not notice a low level. The dipstick in the resevoir should show nearly empty when the engine is stone cold, nearly full after the engine is fully heated up. You may want to have a service shop power flush and fill the cooling system. The chemicals used should break through any blockage.

 

X2 on the rest of your post, but never use any chems if they are not specified by Dodge. There are too many issues with caustic flushing..