When you hooked up the garden hose to the heater core, what was the flow like when coming out? I have a hard time thinking your valve is bad, if you are able to easily flush the heater core.

For flushing the heater core, you could go to an HVAC store, there is a special chemical I used to use for flushing water cooled refrigeration condensers, it was great for getting the scale out of them, and I would think it would work good on a heater core too. Be careful though it is pretty strong stuff.

How is the air flow out of the vents when heating? It could be that the fins are actually plugged with debris as well.

I have a hard time thinking it is a flow problem, when you are able to easily flush it with a garden hose, and both heater hoses are getting hot.

 

The flow was relatively close to the pressure from the water hose, I backflushed it and collected a few gallons of the nasty gunk the first time until it ran clear and then flushed it normally taking about five minutes each way, while isolated from the cooling system of course, and then connected everything. Heat would be great, immaculate even, for a day or so and then it would slowly creep away the next day until you could only get heat on lower blower settings and then none at all. The truck had at one time had A/C but the compressor went bad, and I know there's a valve that stops circulation to the heater core when the A/C is on by means of a vacuum so I have a feeling it might be that once I get up and going something is going wonky and shutting the valve until I flush it again, or it's getting clogged again, but with each respective flush (And I flushed it about five times during the cold months over the course of four months) the water coolant mixture would be cleaner until it was just water and coolant and then water once it ran clear. My problem is I have no idea where the valve I mentioned earlier would be, I've looked for it and I know what one looks like and it's not on the firewall or the inlet of the heater core so that leads me to believe it's under the dash somewhere, I would just like to get some confirmation on that before I tear my dash apart.

 

if there is no valve on the heater core hoses in the engine compartment than your truck doesnt have a heater control valve so theres no reason to tear the dash apart. you said earlier that you would get great heat after you flushed the heater core and would lose that heat over time. when you lost the heat, did you flush the heater core again? if so did you get more crud out? when i did mine, i had great heat for almost 2 months before it plugged on me and i had to flush it out again

 

When I lost the heat I did flush again, but I'd generally lose the heat in about two days time, not a month. It's possible my entire cooling system needs a thorough cleaning, but no, usually it was cleaner with each flush until it was just clean. Is it possible I have an air leak somewhere and air is getting into the heater core?

 

an air leak isnt possible, the cooling system is under pressure when it gets warm. also the heater core isnt the highest point in the cooling system. if air did get in the system it would eventually wind up in the radiator. you said that both hoses to the heater core get warm so coolant is flowing into the heater core. there has to be a blockage, most likely internal as you say you have good heat for a while. if the heater core was plugged with crud on the outside there would be little to no heat transfer to the incoming air so no matter how good the flow was into the heater core you would still get the same amount of heat. the only thing i can think to do is flush out the heater core, then hook the inlet and outlet hoses together going back into the engine, and flush just the engine out with some commercial radiator cleaner and then refill everything with fresh coolant. i hope i helped and apologize for the long post

 

If your cooling system had a leak, it would push coolant out, as the system heated up and pressurized. Once you shut your engine off, the cooling system would cool, and the coolant would retract. As it cools this will cause a vacuum in the system, and will pull coolant from the reservoir into the cooling system. Several situations may happen here causing air to be introduced into the cooling system:

1. Your reservoir runs out of coolant, and it sucks air in.

2. The hose running from your rad to your reservoir tank has a leak in it and is sucking in air.

3. Your rad cap's upper seal is leaking and causing air to get drawn into the system when under vacuum.

The heater core and the hoses going to it are pretty darn close to being the highest point in the system, if they aren't the highest point. I had a nasty air pocket in my heater core that wouldn't work itself out, I had to disconnect the heater core -> water pump hose halfway and dump coolant down it to purge the air out manually.

If both hoses are hot though, I would think you'd have good flow. Have you noticed any coolant leaking, or your reservoir dropping?

 

I've replaced the radiator cap, but I have noticed the resevoir level dropping or fluctuating quite a bit. And I think I may have a slow leak in one of the hoses to the heater core itself, could that cause an air pocket as the truck sits and cools?

 

THe reservoir will raise and lower depending on if the vehicle is hot/cold, that is normal. If you see the reservoir emptying out when checking with the vehicle at the same temperature all the time, that is a problem.

A leak in the cooling system will introduce air in the system. As the system pressurizes the coolant will leak out of the system. Normally if the system reaches a pressure higher than the rad cap, the rad cap will release coolant into the reservoir until the pressure drops below the rad cap release pressure. This coolant that is sent to the reservoir during high pressure periods is then brought back into the cooling system when the coolant cools and creates a vacuum, sucking coolant from the reservoir back into the cooling system.

Now with a leak in the system, the coolant will come out of the leak once it pressurizes, not into the reservoir where it can be reclaimed. When the truck cools it will still pull a vacuum this time though it could suck in from the leak point, which will cause air to be introduced to the system.

Fixing all leaks in your cooling system is important in diagnosing any issues with it. I have to ask, you've had that hose off several times, why not replace it while it was off?

 

I hadn't really noticed the leak until now, as I said I've been busy and moved recently and it sort of fell by the way-side. I'll replace the hose and flush the system and try to dislodge any air in the system and report when I can and the local auto parts store is open.