Rednroll

Based upon your original description of "gunk" being present, I would venture to guess you may have had the Hoat/Oat coolant mixture problem which causes a gel to form. The heater core has the smallest passages throughout the system, so it is likely the gel was blocking the passages in the HC. Reversing the hoses sounds like a good idea. That way you essentially got a reverse flush using hot coolant. However, whatever was in the coolant blocking the HC is now somewhere running through the system.

sjm2011

The gunk did have a gel look and feel to it and the mechanic commented about how much stuff was cleaned out during the flush. I'll do another coolant flush in a couple months to get rid of any remaining residue. I think that if the clog was the casting sand a lot of the other Wrangler owners have had problems with, my core would have been toast and needed replaced.

resharp001

I haven't had to research this, so probably a dumb question.....but if the whole heater core is clogged (implied by inlet temp of 180 and outlet only 90), how does heat get to one side of the vehicle? I'd think neither side would be getting hot air?? Again, I've not had to look at the heater core on mine before....am I missing something obvious?

sjm2011

I watched some youtube videos of other Wrangler owners with the same symptoms changing out their heater cores. When they cut apart the old cores, the driver's side had the blockage. The inlet/outlet tubes are on the passenger's side so there must be some flow even with the restriction. I'm not sure of the flow pattern thru the core but so far it has helped us get heat into the Wrangler without any extra cost.

resharp001

ah, thank you.

Rednroll

Yes, mine was blocked with casting sand.
To add to this, here's what I feel is happening. Even with the majority of my HC caked with casting sand the inlet and outlet hoses had the same temp readings which originally made me think my HC was ok. Even with a brand new HC, what I found is that the driver's side vent will always blow cooler than all the other vents. I found it was 15-20deg F cooler once my heat was back to fully working good. The air has to travel furthest from the HC to reach the driver's side vent than any of the other vents and that air also passes over the blend door to get to that vent (there's very cold air directly behind that blend door in the Northern winter areas), so that's my theory on the reason the driver's side vent is always slightly cooler. Just loss of heat of the air due to thermodynamics of distance traveled and flowing over a cold surface (ie the blend door). Anyways, the heater core is about 12inx8in in area. With a partially clogged heater core the area of the HC which is actually getting hot is drastically reduced (think of it as now having a HC which is 3inx8in in actual area) and therefore the driver side vent will still be coolest but since it's 20deg cooler to begin with, it will be the most noticeable where you will feel cold air blowing out of it. Since the HC is not fully blocked when caked with casting sand, coolant will still pass through it and that is the reason the inlet and outlet hoses will measure the same temp as is what I discovered. The problem is that the hot coolant is no longer able to flow through all the slots inside of the heater core to be able to fill the entire HC. In the example above with the gel, it is likely many more of the tubes inside the core were blocked and is the reason for the inlet/outlet hose temp differences. Gel material is just much more easier to back flush out of the HC stuck in those tubes than casting sand which is packed inside of those tubes.

At least that's my theory and I'm sticking to it. I've had others doubt my theory and when I asked them for a better one they couldn't provide one. As many know I've spent a lot of time diagnosing my heater related problems, and I dissected and inspected my clogged heater core once I removed it, so I've grown pretty familiar with how the heater works in our JKs, so I feel pretty confident with my theory until someone provides a better one.

Rednroll

I've never seen an actual JK HVAC diagram, but based upon the work I've done on my heater system and what I'm describing, I put this simplified diagram together to better show what I think is going on. The driver side vent, the hot air travels through the ducts the furthest and also passes over the blend door area on it's way to reach the driver side vent. Combine that with what sjm2011 described of the HC becoming more clogged on the side closest to the driver side and it all adds up to the least amount of heat from the driver side vent. Basic thermo-dyanamics at work if you ask me.

resharp001

^^^That makes good bit of sense. Got me curious to run check the temps out of each vent now just to see how much lower my driver's vent is.

Rednroll

That's something I'ld be interested to see. No one seems to ever measure all their vents when the heat is working fine. They just notice the driver's side is the coldest when the heat starts performing poorly.

When I finally got things back to working with my heat, I measured all the vents. Heat was 20deg F lower from the driver's vent than all the others. I'm contributing this due to the air having to travel further and it also blows across that section of the ducts where the blend door is located. The way I measured it previously without a thermometer is that I remembered when it was really cold out I would I hold up my hand directly in front of one of the center vents and I had to pull it away after about 30 seconds because my hand was starting to burn. However, when I did that in front of my driver side vent, I could hold my hand there forever. Thus, proving even when it was new the driver's side vent was cooler than the other vents.

Here's my measurements, with a brand new heater core and a full coolant system which has been burped.
Driver's vent=155.5deg F


Center vent (Driver's side=173.7deg F)


Passenger's vent (Hard to read=172.9deg F) *Harbor freight thermometer screen blanks out at 175 deg F.

Rednroll

And Prior to replacing my heater core here were the comparable measurements.

Driver side vent (bad heater core)


Center vent /Driver Side (bad heater core)


Passenger vent (bad heater core)


So in both instances, of good HC vs bad HC there was a 20 deg F difference between the center vent and the driver side vent. With the bad HC the passenger side vent was about 15 deg lower than with the new HC. I would contribute this to the fact that the HC is more clogged on the driver side, so it's performance is down, but not down as far as the center vent and the driver side vent which would be getting more of the air blowing across the HC section more towards the driver side which has more blockage.