My wifes heater has suddenly quit working in her '99 cherokee so I've been doing some reading and from the bits and pieces I've found it doesn't have a heater control valve and the problem is probably in the blend doors. Seems to be a fairly common problem... My question is this: If it doesn't have a HCV does that mean there should be flow to the heater core at all times?? Her's is coming up to temp and the water is circulating so I know the thermostat is good but the heater hoses under the hood are remaining cold. Trying to figure out if it's a clogged HC or if the doors somehow also control flow... Either way I'm not looking forward to tearing out the dash!!! I need to be working on mine, not hers

 

Recheck the heater cores hoses. Both should be hot but a clogged heater core can result in ONE of them being cooler than the other.

What is the coolant temp reading on the dash gauge. Target is the 200-210 range. Cab heat is a byproduct of engine heat so don't want to ignore this basic thing.

A clogged heater core can sometimes be restored to function by backflushing both ways with garden hose pressure.

 

seems like youve got a clogged or misconnected heater hose. not DISconnected. if it was disconnected there would be water everywhere

 

Unless it's a partial blockage won't both hoses be cold?? If there is no flow due to restriction then neither hose should get hot cause water can't circulate in or out.... The only heat in the hoses would be radiant right?
Temp is normal (210) and circulation is good so I guess I'm going to pull the heater hoses and try flushing... I'll update in a few hours hopefully with good news.

 

Well I pulled the heater hoses from the thermostat housing and the water pump and ran water through the HC in both directions and she's clear (no Blockage at all). So I hook the hoses back up, top it off with water and let it run for about 30 minutes... Still no flow through the heater hoses!! They both stay cold, well the one from the thermostat housing heated up about a foot of the hose but that was just radiant heat.... I guess since i'm getting some heating on the hose going to the TH I have water trying to enter there. I know the heater core is clear so I guess my next step is to check the 8-10 inch metal pipe going to the water pump, maybe somehow it's become clogged and is stopping the flow... Anybody have any other ideas?? Is there anything at all in the dash that controls the flow similar to a HCV??? I know it's free flowing when it's not running but maybe something vacuum activated that closes it off when it is??

 

Water pump?

 

Thought crossed my mine but the water seems to be circulating really well everywhere else.... May be my next option though. The fins could be corroded or worn down and causing it not to flow as good as it should but I swear it seems to flow well enough to push water through the heater core and ,as prone to overheating as the I6's are, I would have thought reduced overall flow would have caused some overheating issues....

Also, It quit pretty much over night.... If that helps.

 

Update time: Looks like water pump Yea that makes me happy! I wasn't wanting to pull the dash.... I can change the water pump in under an hour!

I checked everything for clogs and it's all clean. I was pretty sure it wasn't the thermostat but just changed it anyway with no results. Finally, I pulled the heater hose from the thermostat housing and cranked the truck and no flow there so the problem isn't in the HC. The flow just isn't there so I think it pretty much has to be the water pump...Right? Hooked it all back up and turned on the heat and it was cold. Rev and maintain the engine at about 2,500rpm and it get's warm, so I'm assuming the fins are worn and the increase in rpm is making the difference in flow. Just surprises me that I haven't had any overheating issues... Oh well, I'll change it out Wednesday and we'll know for sure If anyone has any other ideas please share! -HR

 

Plus one for the little guy!

 

HighRoller,did the WP end up being your problem?

 

Haven't changed it yet.... Too cold and windy outside. Plan on doing it either tomorrow or fri... Probably Friday when we climb back into the low 50's Having no garage sucks, banging cold knuckles isn't fun I'm 99.9% sure that's it though. I'll be sure to update the thread as soon as I change it

 

When you say that one is cooler than the other.... is the cooler one usually the one from the heater core to the water pump?? ** sorry wasn't trying to jack your thread... but am having the same issue... and that is how my houses are... so was wondering if that was right for the lower hose to be the cooler hose??***

 

The warmer one is the inlet and the cooler one is the outlet usually.

 

That's usually the way it is but really it doesn't make much difference which way coolant flows thru the heater core. While coolant is in the heater core, heat is being removed and blown into the cab so coolant that exits the heater core will be somewhat cooler than the coolant that enters the core. If all is working correctly, both lines will feel hot to the touch, just one a little less.

 

It's fixed!! Turns out it was the heater core.... Flushed it several times the other day with water with no results but I figured I'd do it a few more times today before I pulled the pump and it's working now I flushed it this time with compressed air and used "The works" for calcium, lime, and rust....$1 a bottle at Family dollar and it only took 3 bottles to soak it twice. Let it sit in the HC for about 5 minutes and then flushed it out again with air and repeated, then I flushed it several times with water to be sure all the chemical was out. Pulled the thermostat housing and drilled a 1/16 inch hole at the 12:00 position of the thermostat to let any air through so it wouldn't vapor lock and now she works great!!