please help have a baby and truck has no heat

[eclipsegt123]

hi there i have a 2003 f350 with a 6 liter diesel. Here is my issue. When i start my truck it warms up fine and when i let my truck idle around 1000 rmp it pump good heat. When i drive it starts to cool down. nWhen i stomp on my gas and let off it blows cold until i stop and let it idle around 1000 to 1200 rpm. i topped of my anti-freeze and everything seems normal. my truck coolant temp is around 180 and that is where it is in the summer also. Anyone please help 4 weeks before i can get it into a shop they are all so backed up . sorry for posting it again just very desprete for help

[justinlogue5_0]

I have had two vehiles do this to me and both times it was the heater core gone bad. Mine wasn't leaking so I figured it was just clogged. I disconnected both lines going the heater core and used some aluminum acid flush from a buddy of mine that works on house air units. I just kept pooring this stuff through untill it looked clear again. After that I then flushed it with the garden hose for about 5 mins. Put hoses back on and topped of the antifreeze and the heat would then RUN you out of the vehicle. If that doesn't work then you will just have to replace the core. Hope this helps ya some.