Lukewarm heater

89 XJ, with many, many miles on the clock. During last winter I had a situation where the heater would blow lukewarm air out the vents. It got better, but seemingly at random.

I've flushed the coolant system several times due to broken pressure bottles, then a rad. Replaced all the heater hoses, and made sure to flush the heater core in both directions until I got clear water coming out.

I seem to remember finding something last winter about the blend doors not closing all the way, and allowing cold air to come in along with the heat. I've searched and searched, and can't seem to find the thread any more. Does this sound familiar to anyone? Anyone have any better ideas? First hard frost came in tonight, and we're supposed to have snow before the weekend.....I'm getting tired of driving while wrapped in a blanket.

Snow before the weekend? Supposed to be 65 here on Thursday.

Sometimes you can feel it in the temp control if the door dosen't close all the way in one direction or the other. Mine always felt soft going to the red side, but had a solid thunk when I slid it to the blue. Under the passenger side dash is the cable coming from the control. On it was a spring metal lock clip. I just squeezed the clip and slid it a little bit along the cable and now the door thunks at both temperature extremes.
This was of course the last thing I checked after several flushings of the cooling system and heater core...

There's a vacuum actuated valve inline with the heater core hoses that is supposed to open when you turn on the heat and close when you turn on the AC. It's probably stuck closed for some reason. IIRC it is normally open and uses vacuum to close it. You can fix it so that it's permanently open with a tie wrap or something, or maybe even just disconnecting (and capping) the vacuum hose. It will slightly affect your AC but not significantly.

You can also just delete it and splice the hoses together. But if you use AC in summer, it's better to keep it around, even if you have to manually switch it.

OK....updates. The heater control valve moves like it is supposed to....but I haven't gotten a chance to get in and check out the blend door yet. Heat is still lukewarm.......

I got an interesting idea today from a friend....If there was a problem with my t-stat not functioning properly (being stuck open), could that cause a problem like this? The Jeep doesn't overheat or anything, but it does take quite a few minutes to warm up in the morning.....

Also, I was wondering. The heater hose with the heater control valve....is that the top hose, or the bottom?

Definitely, a stuck thermostat can create the symptoms you are describing; it may be intermittent as well. Replace it; they are cheap; you have to eliminate the basic stuff first. I would recommend a factory OEM 195 stat directly from Jeep as I have had mixed (poor) luck with some of the aftermarket stuff.

if you've flushed cooling system a few times, I'd definitely replace the thermostat-- lots of crud can get freed up during a flush and it could very easily be keeping therm. open.