All,
I have a 1998 Automatic 4x4 4.0L Cherokee with no heat. If the temps are around 40F it will blow warm air. If the temps are around 20-30F I can turn the heat on every five minutes or so and get a burst of warm air. Below 10 F pretty much no heat ever. The temp is always dead center on the gauge. If I do some dounts in the snow, the temp will go up a small amount and the heat will work ok. I have flushed the heater core, replaced thermostat, and made sure the coolant system is full multiple times. The blower motor seems to function properly. The blend door moves as it should, and the region selector works as well. Not sure what else to look at.

 

What thermostat do you have , and have you flushed the heater core ? How hot are the heater hoses ? Cherokees do have a problem plugging up the heater core so it is important to flush and reverse flush the heater core .

 

What's the ACTUAL temperature of the thermostat housing when it's warmed up? (use an IR temp gun). Same with the hoses? It's gotta be 175 or above to get meaningful heat on cold winter days.

My guess is two things:
1) The outside air flapper (door), isn't closing, or is partially closing ...or the foam seal is shot. Basically, even if the heater core were good, it can't overcome too much cold air coming into the cabin. You can check this. On mine, the door was binding and was hanging up internally. But on top of that, the seal (foam) was gone too. Deteriorated from age/elements. The same thing could happen with other doors inside the heater box. Mine is a different year/design than yours however.

2) The heater core is plugged. Permit me to share a story if I may: A particular car I had many yrs ago never threw much if any heat since I had bought it (used). Like you, I checked and tried several things. Eventually, I removed the heater core, flushed it, and even put draino in it and 'baked' it out. It got so hot you couldn't handle it. Put it in, still no heat. The heater core on this vehicle could be done easily via an access plate in the heater box (thankfully!). So, not getting any heat, I removed and 'baked it' again this time with another (different) chemical to dissolve lime/calcium. The core with chemical in it got extremely hot and hissed for a couple hrs, so hot you couldn't touch it. Once it cooled down and chemical reaction waned, I flushed it, reinstalled. Still no noticeable improvement in heat. Eventually I broke down and forked out $250 for a brand new heater core (from the Cadillac dealer -- no aftermarket was avail for this car). I was tight on money at the time due to new kid and bills, ....had been looking for a way to save money. I installed it and guess what? It heated SO DAMN WELL I could practically heat a house! And it continued to heat well for years until I got rid of it.

But before you replace the heater core, try back-flushing again, this time stick the end of the hose right on it (tight) and blow it out in reverse flow (from normal). Don't get too ccrazy with the water pressure though. Check all the vacuum hoses on the heater box door motors too (and the linkages). Same with the outside air damper motor (and linkage) under the pass footwell.

 

I'm guessing you went aftermarket with the thermostat. Get an OEM Mopar, and I bet you'll be fine as it will get up to temperature as it should.

 

Does this mean that the control for the a.c./heat is in the middle horizontally between cold and hot or the gauge for the temperature of the coolant at the head of the block is in the middle of the semi-circle? What temperature thermostat did you buy? Murray makes 160, 180, 192, 195, & 205 deg. F. models for this engine.

You might check your heater control valve (~$20 from O'Reilly's) since you have already flushed the heater core. https://www.autoblog.com/2016/01/08/...control-valve/ You can test the temperature of the inflow and outflow of the heater core.

 

'98 doesn't use a heater control valve.

 

Here OEM/ factory is a better investment. THat's one you don't auto zone..

Agreed. Sounds like his stuff is still getting vacuum from the ball in the front bumper; since the flaps are not stuck for defrost.