1998 ram 1500 with a 360. When it got cold i was getting no heat in the cab. Changed thermostat and now i only get heat when my engine is at 2000rpm or higher. When i stop at a red-light or drive at highway speed with low RPM i get no heat. The fan blows at same speed and the difference in temp change is almost instant. Any help would be greatly appreciated.

 

flush your heater core BOTH ways and see if you have a difference I know its kinda cold to do but you may try to do it in a garage or something. IF your at the light and you rev your engine to 2000 rpms does it get hot again? if so you may have a water pump thats going out on you.

 

+1 water pump

 

You could try your water pump, but if you seach this forum you will see many posts of people with little heat. I have replaced my thermostat and water pump as well as flushing the core several times and still very little heat.

 

a weak radiator cap can cause the problem, because it will not build up pressure... you can take it to autozone and they will test the cap for FREE ( at least the one up the street from does)....... just a thought