heater not working

[chevy2500hd]

I've never seen a coolant valve on these trucks. I would look towards the heater controls and doors within the heater box. Once I had a LPG powered truck and the heater hoses were connected like yours are one to the intake and the other to the water pump and it didn't work that way. It had to return into the radiator just below the cap, so from the intake to the heater core  back to the radiator.

[Greybeard]

chevy2500hd

Yup, I admitted my mistake and realized I was thinking of some work trucks we used at work. I went and refreshed my rotten memory on my truck outside and saw my embarrassment. D'OH! being old and away from some things for a while is a tough pill to swallow at times. I do not and want not to give false information. If I believe something is true in the future I will give that information as such..a belief. But some things were home built back in those days just as today so...

[Irish_Alley]

yes some did put shut offs on the lines just because some of our trucks the fan blows all the time. so they would put controllers or valves just so they wont be blowing hot air into the cab in the summer

[DnStClr]

I keep wondering if the problem could be a collapsed hose going to or coming from the heater core. Installing the radiator cap is causing a pressure change and something in the system isn't reacting correctly.

[thirsty]

You don't have compression going into your coolant system do you? Intake or head gasket maybe.

[Greybeard]

I keep wondering if the problem could be a collapsed hose going to or coming from the heater core. Installing the radiator cap is causing a pressure change and something in the system isn't reacting correctly.

I would agree with this too. The heater core as far I know (that is disclaimer) is a straight through system with no check valves or anything. But as in all liquid systems the flow has to be allowed to go somewhere for it to come from somewhere. The fact it's cooling off indicates that the flow is stopping or the damper door is closing (or moving out of the heat position at any rate).

If the coolant line are hooked up correctly there is a very real possibility I guess (another disclaimer) that the lines could be collapsing as DnStClr suggests. The return line should be connected to the water pump after all which I assume (disclaimer) would cause somewhat of a suction. The cheap attempt to fix this would be to buy quality hose from a reputable source like Napa. (I always say Napa not because I like them but because they are one of the only two auto parts stores in my town. Oreilly's being the other so Napa, as far as I'm concerned, has the better supplies, Oreilly's is where I usually get my stuff from though because they are less expensive). 

[Irish_Alley]

every picture i see has the supply (intake) hose in the bottom. and the return (water pump or rad) on top. but he hasnt been on in a month so maybe he fixed it

[rich weyand]

God, this thread goes on and on!

Yeah, Irish_Alley, we haven't heard from the OP in a while, so maybe he's fixed, but just in case:

From the picture, the hoses to the heater core are hooked up backwards.  This is confirmed by 1) my own truck, which works fine, thank you; and 2) the 1973-1978 Chevy Light Truck Service Manual.

The bottom connection to the heater core is a smaller hose connection.  It is the supply connection into the heater core.  It must be connected to a hose nipple on the intake manifold.

The top connection to the heater core is a larger hose connection.  It is the return connection from the heater core.  It is connected to the water pump nipple, or to a nipple in the top of the radiator reservoir.

The purpose of this is to fill the heater core up from the bottom, so air is forced out the top and back to the system, preventing air from being trapped in the heater core.  Air wants to go up in water, and you want the air out, so up has to be the out direction.

Before you do anything else, re-connect the hoses properly.  You may have other problems, but without the hoses connected properly, who knows what is happening in there.