Heater has hot air when the blower fan is off and im cruisin but seems to blow cold air when i turn the fan on. Any ideas? I did just get back from a road trip and it was cold, the truck was parked for maby four days in an area notorious for packrats, would a rat nest block the heatercore and cause cold air to be blown in? Anyways, any suggestions would be appreciated before i start diggin in to it.

 

That's a good possibility. Also check for low coolant or a defective heater shut off valve.

 

My heater problem ended up being the thermostat.... the engine temp gauge didnt look too cold... about 25% of the total range.... but the heater didnt work worth a squat... i finally figured out that the thermostat had stuck open. The new thermostat rose the gauge to >50 or60% of the gauge range, and the heater worked great.

i had the same problem... heater was warm when there was little or no air moving accross it,, but got colder when the air flow rate was turned up.

 

Before my trip i flushed the cooling system and put in a new thermostat, everything seemed to be working great. The temp gauge moves around a lot but i hear thats normal operation for these rigs.

 

A year ago I had something similar happen to me, drove for 8 hours with no heat, it got a bit chilly, but it ended up being that the heater core was just plugged, works great now.