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Hey guys, I've had my '77 Cherokee since October as my daily and have recently started having a strange problem with my heater. It has worked very well, even in subzero temperatures, and the fan blows pretty well. But recently it has been randomly not getting heat. As in, the blower is still going but I'm getting cold air from the vents. It will do this even after warmed up and having already had heat going. It also has done it from leaving my house to about halfway to work (30 mile trip). Nothing seems to change it, even moving the heat mixer lever. It's done it three times in the last two weeks so I can't easily check for issues. The two most recent times, it started working again after slowing down for a turn. Not sure if that's a clue of any type.

I'm stumped on what it could be without tearing into things but have some potential culprits. Looking for some suggestions from this group if anybody has heard of this issue before. I will be swapping out the radiator soon since I have a leak (does not seem related). When I do that I will be replacing the upper and lower hoses as well as the thermostat as preventative maintenance. If it still does it after a coolant flush I will be pulling the heater core and seeing if there is anything strange going on there. Any thoughts or suggestions much appreciated, it's too cold for this!

What does the inside of your radiator look like? If it has a lot of build up there could be a clog in the heater core.


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What does the inside of your radiator look like? If it has a lot of build up there could be a clog in the heater core.


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Might be a really bad leak and your low on water so it's not going through the heater core. If you go too low on coolant level the heater core doesn't get hot water and no heat.

Get the leak fixed and flush your heater core out with a garden hose.

Should be great.

I've been topping off the coolant every morning so it's not causing any issue. I would assume if there's sludge that it would be a constant problem. The fact that it works perfectly 90% of the time is what makes this so strange to me. I'll definitely be flushing it when I swap out the radiator.

To the Wagoneer experts - what happens if you lose vacuum? Does the heater door close? Possible you could be losing vacuum due to bad hoses or such, and the hot air becomes intermittent.

tgreese wrote:To the Wagoneer experts - what happens if you lose vacuum? Does the heater door close? Possible you could be losing vacuum due to bad hoses or such, and the hot air becomes intermittent.

Heater door is controlled by a cable wire. Vacuum is for switching heat/defrost.

1) are your heater hoses going into the heater core cold, warm, or hot?
2) is your radiator about the same temp?

a partially open thermostat can be the culprit if hoses are cold, and in this season, Indiana is probably not warm.

if the hoses are warm or hot, as said above, a plugged heater core or a leak at the heater core preventing hot coolant from warming up the core...

ShagWagon wrote:

tgreese wrote:To the Wagoneer experts - what happens if you lose vacuum? Does the heater door close? Possible you could be losing vacuum due to bad hoses or such, and the hot air becomes intermittent.

Heater door is controlled by a cable wire. Vacuum is for switching heat/defrost.

The hot/cold blend door is cable operated but the vent selector doors are vacuum operated. If you lose vacuum they will close but you won't get any air blowing anywhere.

Maybe check to see if the blend door cable has fallen off the blend door on the heater box, I've had that happen before.

91blaze wrote:I've been topping off the coolant every morning so it's not causing any issue. I would assume if there's sludge that it would be a constant problem. The fact that it works perfectly 90% of the time is what makes this so strange to me. I'll definitely be flushing it when I swap out the radiator.

Why are you having to top off fluid? Coolant leak? You could be getting air trapped in the heater core.

Sorry for the late reply, it hasn't done it again since I posted so it's hard to test what's going on. Yes, I have been adding coolant every day specifically to keep from getting air in the system. That's why I mentioned it, it's not part of the problem. I appreciate the help guys, I'm guessing it was something clogged that no longer is. Not entirely sure but I'll keep you guys updated if I find anything or it does it again. New radiator coming this weekend so I'll know more then.