Hi all,

So I have been battling a weak heat issue for 2 winters now, I have made improvements but I still have what I think is weak heat and freeze on the highway.

Here is what I have done thus far: Replaced the vacuum reservoir, and all the vacuum lines from the reservoir to the intake manifold, checked the non-return valves, replaced the heater core, replaced the water pump, back flushed my radiator multiple times, changed my thermostat multiple times and finally installed one from the dealer (195 degree), checked full operation of the blend door and motor, and made sure that the vent controls work and that I am getting air flow out of the vents where it should be..

Still I have weak heat or what I now perceive as weak heat because it takes a while for the jeep to warm up, when it does I put the heater control to lower, get on the high way and proceed to freeze the entire way to work.

Any insight or things I have not checked would really help me out! 2 years ago this was not an issue and I was sweating on my ride in.

 

I got a remote start & added seat heaters for the exact same reason. It kicks out heat, just not like my wife's JK or other vehicles I've owned.

 

Two important questions:

1. What is your engine coolant temperature when this is happening?
2. If engine coolant temperature is near where it should be (195-210), then with heat on, the inlet to the heater core should be quite hot to the touch. Is this the case?

Could you provide the year, make, model of the vehicle? It matters.

 

Sorry, it's a '99 Cherokee XJ 4.0.

1. Coolant temp ranges between 195 and 210 (its in-between the 210 mark and the next one down).

2. Both heater core hoses are hot, I can grab onto them for a few seconds before it gets too hot to handle so I know that the heater core isn't clogged.

Just an update as well, I was putting my dash together last night (was checking that there was no vacuum line kinked or not attached etc, after replacing the heater core) and checked the coolant reservoir, it was down a little from the "full" mark but no where near the fill mark so I topped it off, no real change except the coolant temp seems to have gone up a little. I had burped the system as well to make sure there wasn't air in the system.

Thanks for the replies, I am very close to teaching my dog to ride on my lap for the trip in to work but seeing how he is a shepherd, that may make working the clutch difficult!

 

If the coolant is going into the core hot, and coming out very hot as well, that is an indication that the heat is not being extracted from the core much while inside the cabin. Which is a clue that the air is not being routed through the core as expected. If both hoses are hot, I think you'll find your problem inside the vehicle (not under the hood).

 

I had this same issue on one of my Jeeps and it ended up being the temp blend door motor/valve. I know you said you checked it but are you sure that the blend door was closing all the way? I would pull the valve off and manually close the blend door to see if that does anything. And just to be sure we are talking about the same valve, its the one on the passengers side, under the dash around where the center console meets the dash.

 

Thanks for getting back to me. I did pull the motor off the blend door and manually moved the door from lock to lock, marked the stem on the door in the 2 positions, put the whole thing back together and checked to see that the motor powered the door from lock to lock.

Dumb question, but Jordan made me think of this; there couldn't be a way that the heater core was shipped with something blocking the face of it like shrink wrap right? I replaced everything over the summer when I was working on my A/C haha and I can't remember if there was something on the heater core itself.

From the sounds of it, I am going to need to cut through the heater box to get a look at what is going on.

 

If something was physically blocking the surface of the heater core you wouldn't feel warm at all or get any air flow through it. I think you'll find the whole issue lies in the lack of the blend door either functioning or sealing. There is no need to burp the cooling system the design rids itself of air.

 

that's what I figured but I'm trying to think of everything at this point even installation error. Same reason for burping the system...

I am going to open up the heater box with a hole saw and get a look at the blend door in action tomorrow to see what is happening. At least at that point I know that I can cross it off the list.

 

So I finally had the time to cut into the box to take a look at the door and noticed that it seems to seal fine from the hot side and the cold side.

I did notice that when the door went to the hot side that the motor somewhat struggles for the last 1/8" but it seems to seal the door better.

What I did next was to put the door in the middle position and then gently move it around to see if there was any vertical play etc and noticed that the door can shift about 5 degrees one way or the other. This makes me think that there is an issue within the motor.

Could this play in the motor be the reason that I am getting cold air through? I am going to replace the motor/gear box next and see if that is the issue, any additional insight would help as well.

 

yet another update:

I pulled the actuator motor off of the box and opened it up to inspect further and it seems that the motor is fine and that the play is not in the motor/gears so I suspect the blend door itself for the play. I am ordering a blend door tonight and just swapping it

 

OK time to ramp up an old thread! So the situation has not improved in the slightest: Here is what I got going on now: still at 200ish degrees, temp inside the jeep ranges from 110 to 135 or so, I have replaced the following: Water pump (swapped back to the stock last week as well), fan clutch, thermostat (Mopar 195 thermostat), all the hoses, flushed and back flushed everything, bend door, heater core, temperature sensor.

I put a thermocouple on the inlet pipe of the heater core and was reading 165 at the best. Am I missing anything? I asked a local jeep forum and they have been helping me out:

http://forum.northeastjeep.org/index.php?topic=41970.0

any additional voices would be a huge help!

 

Did you investigate the damper which regulates outside air/recirculating air, to ensure the foam is good and it can seal properly? If this door isn't sealing well, you'll get cold outside air coming in which won't even go through the heater core ...and it will not be able to keep up. Common on older Jeeps (including mine).

Are there any holes in the firewall which could allow air to come in? Even a little?

 

I can check that and plug any holes in the firewall, I may need to take the heater box back out to check the seal on things, are you referring to the inlet at the cowl?. My major concern is that I all of this started when I replaced the water pump and thermostat almost 2 years ago am still reading a low temperature at the gauge. After swapping the temp sensor I am still reading lower than normal operating temp and that I am getting 165 to the core. Also, forgot to mention that for a while I had a 205 thermostat in there and had the same results.

Any late gen XJ guys willing to measure the temp they have in the cab and let me know what I should be shooting for?

 

While it's possible that the face of the core itself could be blocked by decomposed reticulated foam with an inlet temp of 165 you're only going to see luke-warm heat regardless.
Why was the 205° thermostat in place "for a while"? What's in it's place now?
With a 195° thermostat installed do you see anything close to 200° at the thermostat housing? Have you tried measuring temps with an IR temp gun?