Dodge Repair: 1997 dodge 3500 v10 heater

Question
in cold weather 0 deg. or colder (dec. in minn.) my heater blows medium heat until I slow down or let idle then it turns cold until I get back on the road then goes back to med temp. if outside temp gets 5 or 10 deg. it blows hot air but still turns cold when I stop or let idle. can you help?

Answer
Hi Les,
I think the problem may be that there is insufficient flow of hot engine coolant through the heater core in the cabin. That is most noticed when the rpm of the engine is low. This could either be due to the coolant not being full up (is it showing some in the overflow bottle which would suggest that there is enough?) or possibly an air bubble in the circuit that carries coolant to the core and back to the engine (was the system drained and refilled recently?) or possibly there is a build-up of crud in the heater core tubes. You could verify that there is sufficient coolant, then feel the two hoses that go through the fire wall to the heater core in the cabin after you have driven it for a bit. The two hoses should feel about equally hot if the flow through the core is good. If not, then you might  disconnect the hoses at the firewall and flush domestic water from a hose through the core and out, in both directions, to clean out the crud in the core. Then reconnect the hoses and refill the coolant as described in the manual (which I don't have for '97, my oldest is for the '04 model year). I can tell you the instructions for the '04 year if you don't have any instructions on refilling.
Roland