I'm fighting this problem now... I think the blend door/actuator is behind the glove compartment, I'm going to see if I have movement there and then move to flushing the heater core.

Are there any good troubleshooting guides or diagrams for this?

 

I solved my problem, but not necessarily in the best way.

I pulled the heater hoses and blew out the heater core with water and then compressed air, it seemed very clear, hooked things back up and burped and burped the system and no luck. Same problem, occasionally I would get warm air out of the far left vent.

Then I checked the rear heater, no hot air their either. At that point I decided to eliminate the rear heater from the bundle of snakes next to the transmission. After getting rid of the tees in the line to go to the rear heater I now have heat. after disconnecting the rear heater, I blew out the lines to the rear heater core and it did not have new coolant in it, so there wasn't good flow back there.

I still think this was stull a burping issue, but but getting rid of the tees and forcing all water through the front core, I managed to get the bubble out. I wonder if I reconnected the rear heat, but put the 2 cores in series if it would help. But I don't really need rear heat.

The heat and defrost worked well on a sub freezing day, so I'm calling it fixed.

 

I am curious if your problem is still fixed, as I have the same issue on my 2003 Montero limited and, as you said, mechanics are scratching their heads. The mechanic I took it to also suggested cutting off the rear heat lines/heater core to simplify the system. He also cleaned out the lines in the front. The heat did improve, but it is nowhere close to where it should be. Can you provide any additional information as to what you did? Did you replace the coolant lines? I am just not sure I understand what you did. Many thanks!

 

My heater wasn't working good but I had used some of that barqs radiator stop leak until i installed a new one, which worked very well as far as stopping the leak. After i installed the new radiator, I wasn't getting any heat so I ran the radiator without the cap on for about 15 minutes or so. I also realized the upper radiator hose needed replaced. So those two simple things completely solved my issue. I have super hot heat now. I originally thought that stop leak plugged up my heater core but that wasn't the case at all.

 

Having exact same issues, What is this blend door? how do I confirm if it is the culprit? I was originally thinking it is not burping, and have been trying several different ways to burb, still cold on passenger side.

 

Blend doors is the little gates inside the climate control system that direct air one way or the other. It is possible for them to get stuck. Do you get actual airflow from cold vents, or just no air coming at all?

 

Air flow not the problem, lots of air flow. Problem is trying to heat the cabin up. air is hot from the left side, and get colder as you go across the dash/vents to the right side. Might still need to burp the cooling system again, (for the 5th time).
I'd think the blend door would allow even temperature across all vents no matter what position it was stuck. if it was stuck in the cold...cold air from all vents. Stuck in the warm position.... warm air from all vents. Stuck in the hot position,,,, hot air from all vents. I am not gettting this, but hot on one side, and gradually colder as I go across the vent to the right.

 

So has anyone fixed that issue? I have been flushing the coolant system, flushed heater core, cleaned inside temp sensor, changed thermostat. No changes at all.

 

I'm assuming you're having the uneven distribution of heat issue like you quoted. The problem is likely air trapped in the radiator or possibly the dash wasn't reassembled correctly leading to air escaping the ventilation ducts in places it shouldn't. To eliminate possible air bubbles, get yourself something like a no spill funnel kit that can connect to the radiator fill port and put some antifreeze in it (about 1/4th full), jack up the front end so the radiator fill port is easily the highest point in the system, start the engine and let it get up to operating temperature, turn on the heat to full blast and Start "bleeding" the system of air by gently increasing the rpm of the engine up and down a thousand or so rpms. This can be done in the drivers seat or by manually moving the throttle assembly(wear gloves, it can get uncomfortably hot). Watch as bubbles start burping their way out of the system and keep an eye on the fluid level in the funnel as you may need to top it off at points. Once you stop frequently seeing bubbles coming out, begin increasing the rpm of the engine. I find the best results at this point come from quickly bringing rpms up to 4500 or so and dropping it over and over. Once you do this a few times, switch back to the gentle method. At this point the air coming out of the vents should be significantly hotter than before.

Perform this until you are sure the system is bled correctly. If the issue persists after all this, look into taking the dash apart and reassembling correctly. There is a particularly pesky piece that sits directly atop the heater box that can get all ****-eyed when installing the dash if you're not vigilant.

Edit:
The insulation around the connecting points of the ventilation system and blend door can start falling apart with age, which can contribute to the uneven distribution issue. You can replace this material with padded weather stripping or a similar material, I'll leave it up to you what to use as there are many solutions to this.

 

I have to say - I never in my life had an issue with bleeding air form my Montero cooling system. Not when replacing fluid, not when I replaced various hoses (including hoses to the cabin heater core), not even when I pulled and rebuild the engine. Not once in all these times did I have an issue with bleeding air out of the system.