I've searched and tried just about everything. 74 f100, 5.0, non-a/c. Truck and heater hoses are perfect temp on inlet and outlet. Doors in heater box function properly. But system only blows cold air no matter what I try. Almost at my wits end. Open to ideas and suggestions on what to check and try. Brand new heater core as well and no air in the system.

 

Welcome to the club. I have had the same prob for over a year. I live in S.cal., so it's not that big of a deal, but having a heating would still be nice. I have tried bleeding the system, and every other remedy recommended by forum members. Nothing has resolved the problem. To my thinking, if hot coolant is filling the heater core, and air is being pushed through it, then warm/hot air should be coming out. And yet...nothing but nice cool air. Sorry I don't have a solution for you.

 

I'd use one pf those laser temperature guns and check the actual temp of both hoses coming in and out f the heater core.
And the thermostat housing with the engine up to operating temperature.

 

Tried that. Heater hoses get about 185ish and tstat housing gets to around 205 give or take a few degrees

 

Make sure the water valve is open.

 

We used to have cars come in to the shop this time of year with same issue, but those were typically GM vehicles running that orange dex cool stuff that liked to gel up, especially when someone mixed standard green coolant with it.

The fix was disconnect heater hoses at the heater core and blow compressed air through the heater core in reverse direction the coolant flows. You would see chunks of stuff come out. Reconnect heater hoses and they would get heat back. On my 351M the line from the engine near thermostat is the line out to the heater core, and the line connected to the water pump is the return line from the heater core.

I'm not saying this is the issue with yours, but if you've checked that you have heated coolant up to the heater core, the blower fan is working and the blend door opens to allow heated air to flow into the cabin, I think it would try flushing the heater core and see what happens. If the heater hoses are old, they may have started to deteriorate on the inside and small rubber flakes are plugging the heater core, or just after years of use/sitting the passages have clogged up. If there was ever any "stop leak" added to the cooling system that could have restricted flow in the heater core too.

It's always possible the fins are plugged up with dirt and debris that has accumulated over the years and though the fan is running, no or very little air is actually passing through the heater core itself. If the flush doesn't work, it might be time to pull heater box and take a closer look at the heater core. -EDIT- Nevermind, reread your post and see that you replaced the heater core already so I doubt the external fins are plugged up. It is possible it is plugged internally though, even new, if the hoses weren't replaced and they are falling apart from the inside it could have plugged up right away.

 

Have you verified that the blend door is moving and not just the cable?

 

I am glad to see all the responses and helpful suggestions. With me, and I hope I am not hijacking this thread, I removed the entire heater and rebuilt the entire thing. While off, I confirmed that the two internal 'doors' were moving. I have also removed the 'out' hose from the heater core while running to confirm flow. Great flow.

 

Living in Montana it gets rather cold as I am sure you can imagine. I did drive my current '79 through winter last year and the heater worked okay until it got below about 20*F. I ended up placing cardboard in front of the radiator and that helped a lot, especially when temps dipped teens and 20's below zero.

Years ago in a previous '79 I had, I would swap thermostats out when I changed my tune for winter and add a winter cover. I even modified the heater box to fit the high output heater core when the original failed and all I could find in-stock locally was the HO version. The HO heater core certainly worked better, but when I had the 160*F thermostat the heater was only adequate down to 30-35*F or so. I would swap with either a 195 or 205 thermostat, don't recall which one, I might have tried both. The higher temp thermostat and addition of the cover over the radiator meant my heater worked fairly well down to -20*F or so. Once it gets below -20 everything struggles to stay warm, but you're so bundled up when you step outside anyway you can usually stay warm in the cab. Biggest issue then it keeping the windows defrosted.

It appears OP is in North Dakota so he will likely have the same issues to consider.

 

I disconnected the cable and can move the little lever, I can feel slight resistance so I assume the door is moving, but will likely have to pull it all out and apart and verify. I've also tried removing one hose to verify flow and it is definitely flowing good. I've even tried switching the hoses(which are new) to see if maybe I had something backwards but that didn't make a difference.

 

Perhaps it is something as simply as out of adjustment control cables. Especially on the blend door side. If it doesn't close completely you will have fresh air from the cowl bypassing the heater core and even a small leak would impact performance. There is a lot more resistance going through the heater core than around the blend door and air, like water, will take the path of least resistance.

 

Has it ever had heat when you initially put it all back together w/new heater core and other new parts? If yes, then what has changed to make for now no heat. So look at what could of come unattached, disconnected. What is not working right, flap door is controlled by cables, so I would start there. Any chance any thing got down the defroster vent and into the box and jammed a door?