Hi guys

Now when the weather has getting colder, I noticed that my 5.9 is not blowing hot air in the cabin, only when I rev up the engine around 3k or more. Then hot air comes immediately.

It sounds to me like the water pump blades are gone, right?
Or is it termostat?

Engine temp is good, I also bleed the system, no change.

Heater core hoses are like they should - going in is hotter then coming out.

Have no leaks, pressure tested the system also.

But have over 420k km on the car, and have no idea when the water pump was replaced.

I wonder if the heater core is partially plugged. Before you actually replace anything, take the heater core hoses off and use a garden hose to backflush the system (shoot the hose into the core's "out" nipple). You can make a little easier by putting lengths of hose onto the nipple. Direct the flush water into a bucket so you can inspect. You can also backtrace the hose that goes to core input... take it off at whatever connection you find (where it originates, somewhere around the t-stat housing) and look for obstructions in there. To eliminate variables, be sure both hoses are flowing well.

Not saying this is the issue, but it's a logical cause.

I'm not a mechanic, but I would think that if there was an issue with the water pump then you would have an overheat issue?

DennisG01 - thanks for your input, I should maybe re-flow flush it. It would be the "no-parts needed try".

But why I have instant really hot air when the rpms are high, if its clogged it should not matter.

If it's partially clogged, then there may not be enough force/flow rate from the coolant to overcome the clog at idle. This still may not be the issue, but in my "backyard DIY" mind, it's a very feasible explanation. Remember, the clog could be up at the t-stat housing, too (wherever the hose originates).

Doing the re-flow flush right now.

Did not see any crap in the container tho.

Put some "radiator flush" inside the cabin radiator right now, waiting for about an hour and then flush it out again.

Updates coming soon.

Update: But everything back together, and bleeded the system - there was hot air again on idle

Let it cool and will go for a drive soon, I hope its fixed now.

Update2: its not perfect heat like it should (my 5.2 is much hotter), but now I know that I need to replace the heater core in summer

Well, at least it's gotten better and should hopefully get you through the winter. You might try flushing it again - if you're up to it. Whether it will help, or not, who knows - but certainly can't hurt.

Edit: And, try flushing the heater core both ways for good luck.

If the water pump was bad you'd overheat.

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Having the same problem. Got the heater core replaced and thermostat as well and getting cold air instead of hot on idle. But when I drive I get heat instantly.. Took it today to mechanic and he recalibrated the actuators, when he turned the Jeep on/off he noticed he lost calibration to the actuators. And isn’t sure what the cause is..

Sounds to me like your low on water . But I'd check or replace all the parts connected .
...
Hum I can't believe I left that response at that wording ..
SO Mine is doing the same thing But I've replaced my heater core. & I have no heat coming out . Plenty of air . & I've replaced 2 doors in the dash. & no heat .
.. & A new water pump & It's full on water .
... So If you fix yours let me know .
I wonder if I forgot to save the rest of my answer.

I did it both ways, and many many many times yesterday

My friend just had the same cabin air cold problem on the WK diesel.

Replaced the stat and flushed the heater core, but nothing changed. Engine temp all normal, no overheat.

So he pulled the water pump, plastic blades, all gone. Replaced with metal version pump. Now all good and cabin is hot.

Thats why I suspected the pump myself

Diesels naturally run cooler, though. So it may not be a completely apples-to-apples comparison. I'm certainly not implying that the water pump is NOT an issue in your case - it could be. Maybe it's just partially buggahed up - but the fact that flushing things noticeably helped leads back to clogs behind the issue.

But back to the pump vanes... see if I'm on the right track here... see if what I'm thinking makes sense... if part of a vane broke off and it was small enough, it might pass into a cooling system passageway (blockage, again). But I would also imagine that if a vane broke, it would end up taking the others with it as the pump spun around (sort of like a blender). And in that case... you would end up with no vanes left... no cooling... and the engine would overheat.

Did you check where the heater core connects by the t-stat? I once found a blockage there from a shop that used too much silicone sealant and the extra sealant all collected there, clogging the connection point of the heater core.

Water pump vanes DO wear out. I've seen it in industrial pumps. Even water will wear something out, check out the Grand Canyon. At 420KM it is a possibility. That being said usually the seals will go way before the vanes.

ZJ has metal blades.
Since limiteds with automatic climate control are not vacuum activated, it's quite possible that the actual controls are bad.

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When I disconnected the hose going to the core, i flushed also both sides of it.

I plan to do a full flush and pump and stat - but in spring