Its like this. No heat, I mean who ever heard of this, heaters always work don't they?? Anyway pulled hoses from engine and connected a garden hose to flush out the heater core. Ran water forward and backward through the heater core until the water flowed very nice, however there seemed to be to much back pressure to my thinking, but had nothing to compare it against. The water flowed nicely both directions using a garden hose. Some gook and rust bits came out at first, but cleared up after awhile. Hoses are clear, so connect everything back up and fire up the boiler. Input hose getting hot output hose warming up also. Turn on heat controls and turn on the blower, good I feel heat, what,!!! Its cooling down??? Input hose Hot, return hose cooling down. Seems like water is staying in heater core to long and exchanging the heat with the blower and no new hot water to keep the heat exchanger hot (heater core that is). Now what??? Pull hoses again, flush and backflush, good flow of water both directions. Maybe that stupil little metal return tube is plugged up or something. Well that thing is about $70.00 from Dorman. Seems to be ok, I mean water flows in and out of it with garden hose. Wonder if the Impeller blades rotted off the water pump. I mean its just a heater isn't it??? Ok you guys whats the answer, I am sure someone ran into this in the past. I did not take the metal heater return tube off the top of the water pump, and I did not remove the water pump. The truck temp seems ok, thermostat seems to be operating as it should.
Thanks
Darryl...

 

the check the hoses / heater/ and lines ... take off one hose and connect the garden hose to it...... push water thru the hose / heater/ back to the motor/ thru the system and out of the fitting on the block.... then reverse and do it...... as you mention, BOTH hoses should be hot... or hot and warm if your just idling on a cold day...... there have been occurances of impeller spinning on the pump shaft, or impeller getting eroded and not pumping to capacity....... check everything else first.

 

Well… did you figure it out? Mine is doing the exact same thing

 

Clogged heater core, dash will have to come out.

 

Oh! crap this post is close to 5 years old so this is for fiddle Put the garden hose in the radiator and use the pump to do your testing. start with removing the heater return and get up to temp. Then work your way back to the engine. If your heater core has rust then so does the rest of the system. Flush the entire system. You should start a new post with your truck info. and what you have done so far.
Airborne 82nd.