hey dudes i have an issue with my diesel it wont blow really hot air. i changed the thermostat, flushed out the coolant system reversed it also put in new antifreeze. it wasn't even dirty. i was thinking when it is up to operating temp maybe the doohicky that controls the water temp is not opening all the way or. may be where the hot coolant line comes off the engine is at the half way point on the head ,when from past experience it should be beside the thermostat no. ive had this truck for 2 years now and it comes to the point where i wont drive it in the winter cause its so cold just warm air not really hot like the way it should be now thats springs hear it still warm not hot. even at warm outside temps.
any one have a similar problem with there wheels oh yeah i have a 1999 diesel

 

Heater core could plugged

 

Simple test !

Bring the truck up to running temp, and grab one side of the heater core lines, they should both be hot !

If not, it's a heater core... if they are both hot, then it's more than likely a " Blend " door issue, does it get cold air ( A/C ) when you ask for it ?

 

I agree, I had the same problem and it turned out that the heater core was totaly screwed. Make sure to check for any leaks inside the cab, that will tell you for sure that its the core.

 

heater core definately

 

Ditto, heater core or blend air door, most likely blend air door.