Basically what the subject says..

I can turn the temp to FULL hot air with the dash vents, or any other setting (not AC) and the heat will work sometimes, and not work other times.

It isn't an issue of not being warmed up, because this happened even when pulling approx 12,000lbs.

I really need to get this fixed, as you know, its COLD in WI!

 

Have you checked the level and condition of the coolant?

 

Coolant is new. The level is good.

 

When the truck is cold start it up. If it is cold where you are then put something in front of the radiator to cut down the airflow accross the radiator. Monitor the two hoses that go into the heater core from the engine compartment. There is no shutoff valve so the coolant is always running through. If you have one hose heating up and the other not or only a little you have a partially or fully clogged heater core. If they both heat up it could then be the blend door not fully seating or the foam seal on the blend door is bad. I tend not to believe that this is the problem as if you have not touched the heater control and have left it on the same setting then it can't open and close itself. Check the heater core hoses first to see if they are both heating up at basically the same rate. If they are and your coolant is ok then it has to be something in dash. Hard to believe that it would be clogged finns on the heater core as the problem is intermittent. It is unlikely that the finns get clogged and declogged and then clogged and declogged, etc....

 

If you turn the temp all the way to cool, then to hot very quickly, it will get hot for a split second, then get a bit cooler when you hit the max heat setting.
It won't stay HOT or at least "full heat" at any setting though.

 

COuld the blend door be returning to default position after the initial move to the heat setting? I haven't inspected mine to see how it operates, but have had several problems with 90's model chebby pickups with the servos that open and close the doors causing problems like the one you described.

 

Could be the cable to the blend door. Mine was broken inside and would sometimes work and sometimes not. Only about $25 from Dodge, but a major PIA to change. Good luck trying to get at the door end of it.