99 XJ no heat

Ok so my cousin picked up a 99 XJ 4.0L auto 4x4 that supposedly had a bad head that was causing overheating. Well we have not been able to get it to overheat and we have flushed, filled, and "burped" the air out of the cooling system several times. We also have not seen any signs of a cracked head so we think it was a bad diagnosis or the mechanic was trying to rip the PO off. Either way we got a great deal on an XJ.

But we do have an issue with the HVAC system. We do not get any heat. I was told that the air blows out of all the vents so it will do whatever setting you put it at and not just defaulting to the defroster. The A/C works as it will get very cold. It just will not get hotter than the air outside that it is drawing in. As far as we can tell the heater core is not clogged since both the supply line heater hose and return line heater hose get hot. I thought to disconnect both hoses and connect a garden hose to the supply fitting on the heater core to see if water came out the return side to really see if it was clogged but so far I have not seen any evidence that deemed this necessary.

So now I am wondering if the blend door is malfunctioning? I have not heard of blend doors quit working. I also am not sure if the blend door is operated mechanically or if it runs off the engine vacuum?

Any ideas here would be great as I am a little stumped.

it runs off of engine vacuum and I have seen the actuator arm break.

I did some more searching and found a link that showed a FSM image of the blend door motor and it said it was electric. I have a digital copy of the FSM so I checked it and sure enough it is a little electric motor. So I am going to check its condition.

I did get a little seat time in that XJ today and the air did finally get just a little bit warm so I am leaning towards a clogged heater core maybe.

We also discovered that the passenger side upper vent has no air coming out of it while the two in the center and the driver side do blow air. What would cause one vent to not blow?

muduck18 said:

it runs off of engine vacuum and I have seen the actuator arm break.

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As far as i know the 99 has a electric blend door for the heater. I have a 99. The little motor motor in the servo box can get corroded from condensation, and needs to be taken apart and cleaned (to clean mine It was simply just spinning it by hand to free up the internals). I drilled a few small holes in the bottom of the servo box to allow any moister to drain out. Jeep wanted over 275$ for a new unit so that was way out of the option.

With the the key in the ignition to run (not started) turn the fan to low and turn the temp knob to another position. You should hear the faint sound of the blend door servo making some sort of a mechanical sound, if not then its seized, fried, other.

NOTE: If you take this box off it is mounted by 3 8mm bolts at the bottom of the hvac box in the passener footwell. When you take the box apart to free up the dinky electric motor DO NOT ALLOW THE GEARS TO COME OUT. I know this because it took me about 1 hour of playing with the damn thing to get them re-calibrated to work properly.

I have the same problem. I flushed the heater core and replaced the blend door....still no heat

ktwalker01 said:

As far as i know the 99 has a electric blend door for the heater. I have a 99. The little motor motor in the servo box can get corroded from condensation, and needs to be taken apart and cleaned (to clean mine It was simply just spinning it by hand to free up the internals). I drilled a few small holes in the bottom of the servo box to allow any moister to drain out. Jeep wanted over 275$ for a new unit so that was way out of the option.

With the the key in the ignition to run (not started) turn the fan to low and turn the temp knob to another position. You should hear the faint sound of the blend door servo making some sort of a mechanical sound, if not then its seized, fried, other.

NOTE: If you take this box off it is mounted by 3 8mm bolts at the bottom of the hvac box in the passener footwell. When you take the box apart to free up the dinky electric motor DO NOT ALLOW THE GEARS TO COME OUT. I know this because it took me about 1 hour of playing with the damn thing to get them re-calibrated to work properly.

Click to expand...

Yeah. I was a bit hasty. The doors for the mix are all vac, it sounds like the heat/recirc/vent door is electric. Good to know. My defroster one is broken. And the ARM is off. I never checked the others.

You can see the blend door- on the passenger side kick panel if you take off the plastic kick cover held on by 4 bolt/screws- key on- twist the temp cold to warm and you see the worm drive move.

muduck18 said:

... The doors for the mix are all vac, it sounds like the heat/recirc/vent door is electric. ....

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The blend door is the door that blends heat with AC or air to set the temperature of the air.

On my 2001 the blend door is the one that is operated by an electric servo. The doors that direct the output to the various vents, or select the source (outside or recirculate) are vacuum-operated.

Here is the link to a photo showing the location of the blend door servo:
http://i.imgur.com/51jFs.jpg

Did you find the problem?

I finally fixed mine...heater core was plugged. even though i had already flushed it once, i guess it wasn't enough. This time around i blew it out with an air compressor instead of a garden hose, worked much better. I also filled the heater core with CLR and let it sit in there for about 30 mins. Heater is nice and hot now!

Great thanks, I will do the same this weekend. How much water did you push through to flush out the clr out? Or just air?

Just out of curiosity, what came out of the core? Gunky stuff?

I blew it out with air compressor first. A bunch of brown gunk came out. then i poured in clr and let sit. Then blew out with air compressor again. A butt load of brown gunk came out. Then i poured clr in again and after sitting i flushed out with garden hose on high. I switched flow from inlet to outlet back and forth for probably 10 minutes. Heater is hot now!