MrChaos wrote:
here are the main culperits of the lazy heat issue for post 96 XJ's
naxja said:
Newer Cherokees do not have a heater control valve. Low heat can be caused by a lazy thermostat, a clogged heater core, a failing air blend door, or a water pump with a poor flow rate.
the only thing I see on that list that hasn't been addressed is the water pump which is only a 1-2hr job on a XJ/MJ iirc.
The first thing I did was change the water pump on mine. No change. At least like you said it was a fairly simple job.
Was the replacement heater core OE or aftermarket? Some cheap aftermarket units just don't work well. I ran into this with a mustang. Turned out the aftermarket unit had fins like a normal radiator where as the OE unit had very fine fins. After replacing the heater core for a 2nd time and using a OE replacement. (at 2x the cost) it fixed the problem. I was a bit annoyed as it required taking the entire dash out and removal of the stearing column. Inspection of the two cores found that the oe was a bit thicker an fit alot snugger in the box. And as mentioned previously the fin design of the core was 100 times denser. Once replaced the heat was back to normal.
MrChaos wrote:
here are the main culperits of the lazy heat issue for post 96 XJ's
naxja said:
Newer Cherokees do not have a heater control valve. Low heat can be caused by a lazy thermostat, a clogged heater core, a failing air blend door, or a water pump with a poor flow rate.
the only thing I see on that list that hasn't been addressed is the water pump which is only a 1-2hr job on a XJ/MJ iirc.
Could a water pump be lazy enough to give me E36 M3ty heat, but still cause no other effects at all to the general cooling of the vehicle? After all the threads i see about people having cooling issues in summer, i'm actually quite impressed with this thing's ability to stay exactly where it should in triple digits using this tiny little radiator.
dean1484 wrote:
Was the replacement heater core OE or aftermarket? Some cheap aftermarket units just don't work well. I ran into this with a mustang. Turned out the aftermarket unit had fins like a normal radiator where as the OE unit had very fine fins. After replacing the heater core for a 2nd time and using a OE replacement. (at 2x the cost) it fixed the problem. I was a bit annoyed as it required taking the entire dash out and removal of the stearing column. Inspection of the two cores found that the oe was a bit thicker an fit alot snugger in the box. And as mentioned previously the fin design of the core was 100 times denser. Once replaced the heat was back to normal.
Probably aftermarket. The radiator had been done around the same time and it was a Spectra. I'd expect the heater core to be the same brand.
I think what i'm going to do this weekend is get back into the blend door and make sure it's truly hitting each extreme the way it should, then check temp on the inlet/outlet heater core hoses.
If all that checks out, i'll just have to dump it over to my stepfather in law and have him rip the dash out to look. I don't have time to do the job myself right now. I'll just have to tell him to make sure he remembers to re-install the foam sealer piece that goes in that firewall hole where the hoses pass through.
You know... because he forgot on the red 98 when he did the job. 
I had an issue with the A/C on my '06 Dodge Truck during the summer. The recirc door broke, and when it fell it just about covered the blower motor. I'd get a whole lot of noise when the fan was on high, but little airflow. I dropped the blower motor, pulled out the broken recirc door, and had working A/C again (albeit without the recirc function). Not sure if any of this is pertinent to the XJ's system design, but just thought I'd throw it out there as something to eliminate before you take on some unpleasant tasks (heater core).
Cover up the fresh air inlet at the cowl area so outside air does not get into the HVAC system. Then do your heat test and see if you have heat. If that gives you heat you know you have an airflow problem inside the heater box.
Let us know...
If the heater core is restricted, or the water pump is weak, there will be a noticeable difference in temperature at the outlet between idle speed and 2500rpm.
Drawing outside air, or having the compressor cycling won't make any significant difference to the heat output on a car with decent coolant flow.
Your statement about the heater core being changed also raises flags for me. I have seen situations where the foam seal around the core wasn't replaced properly, or the core was a bit different, and the cold air will blow around the outside edge of the core. If you are confident the blend door is now moving stop to stop, I'd say its heater core time. Make sure you get to speak to the guy doing the job, so you can explain your concerns directly.
On my 99 xj the issue was fixed when I flushed the core, if yours is flowing coolant well after you flushed it then it's about your air flow.
Check that the door goes from end to end and if it does start looking for where that air flow it's going instead
It's either not going through the core or not getting directed to the vents
Can you have a look at a core like you have vs oem? To rule out built quality. That would be a pain.
Good luck.
Streetwiseguy wrote:
If the heater core is restricted, or the water pump is weak, there will be a noticeable difference in temperature at the outlet between idle speed and 2500rpm.
Drawing outside air, or having the compressor cycling won't make any significant difference to the heat output on a car with decent coolant flow.
Your statement about the heater core being changed also raises flags for me. I have seen situations where the foam seal around the core wasn't replaced properly, or the core was a bit different, and the cold air will blow around the outside edge of the core. If you are confident the blend door is now moving stop to stop, I'd say its heater core time. Make sure you get to speak to the guy doing the job, so you can explain your concerns directly.
No difference in heat at any rpm.
So yeah. I'm not liking where this is going. 
can you wiggle your hand up under the dash and put your hand of the box that the heater core sits in.
If you can, is it warm?
Maybe? I'm not sure where that is.
Is it just behind where my blend door was?
Oh yeah, no way in hell i'm getting back there with the dash in. Le sigh.
Are you getting plenty of air out of the vents just not warm air?
what is book time on a XJ heater core? 6 or 9 hours?
I think it's plenty of air, but unsure if it's getting quite the same as the 98 is.
Worth mentioning that the Jeep in question has a different hvac system than what's pictured above, which is why i haven't been worrying about the small flow difference between the 98 and the 00.
Sooo, what kind of camera do you have that you can thread down thru a vent to see inside?
There are two water pumps for 4.0's: normal and reverse rotation. Might want to verify yours has the right one.
How do i tell looking at it from the engine bay?
What would be symptoms of having the wrong one? (And what sort of idiot would put the wrong one on?) I highly doubt it has the wrong one. We're the 3rd owners, and the guy we got it from wouldn't have done that.
And I am not saying this is your issue... On my VWs, the heater doors had some sort of foam on them to let some fresh air mix with the heated air. Unfortunately, over time the foam disintegrates and leaves big open holes in the heater doors like this.
Which causes too much cold outside air to mix with the heated air. Causing low heat inside the car.
No foam on doors inside cherokee. Both doors are just plastic fins.
New hot/cold blend door installed sunday is metal, so that's cool.
iirc i might have linked the picture of a pre 96. But the 98 and 00 should have the same system but not the one pictured..
MrChaos wrote:
iirc i might have linked the picture of a pre 96. But the 98 and 00 should have the same system.
Nope. 98 is still cable. 2000 is not.
In reply to Swank Force One:
swap in a 98 dash plus system to fix issue?
