Have a 1990 Cherokee Laredo with straight six has no heat. Changed thermostat first thing last winter no change. I tried a back flush on it and let sit with clr in it then flushed with water till clear. No real noticeable change. The line going into heater core is hot and line coming out feels cool. If I run the engine up to about 4000 rpm it will blow hot air but as soon as I let off gas it will cool right back down. Just curious if anybody had any ideas before I change the heater core which I'm dreading.

 

I have the same problem
And I was gonna go to the heater core. I may hold off and just gut all my heater/a/c stuff.

 

I can't remember but I think I may have put a thermostat rated at 160 instead of the 195 I don't know if that might have anything to do with it. But the heater hose are still uneven temps

 

I'll bet you its a vacuum issue; considering the rev symptom. I don't know how they are on the older XJ's but maybe your blender valve isn't staying open. On the later generations its a cable so it surely wouldn't be the case.

 

f;ush/backflush the heater core

 

The blender flap is controlled by cable. I'm going to put the 195 thermostat in it is only four bucks. Then I'll try flushing it again. There is a vacuum line that is blue that isn't hooked to anything and i can't tell where it was supposed to go. If anyone has any pictures or diagrams of the vacuum lines for the heat would be awesome.

 

question? does heat come from defrost when you select that?

if so, then the "blue" vacuum line has come loose or split on its way to one of the vacuum 'valves' behind the dash and heatercore.....

not allowing heat to be selected via the door mechanisms.

somewhere, and I cannot remember the post but on NAXJA forum someone had a diagram......of that system..

sorry I couldn't be of more help sir.

cpnwrench

 

The blue isn't hooked up at all

 

vaccum leak! thats your problem

 

You would you happen to have a pic of under the hood I'm not sure where to hook it up to

 

If I'm not mistaken, the blue is a vent for the CAD system (4x4)

I have a heat problem also. It hot on the bottom and cold out the other vents, including the defrost.

I have a tan line running to the heater control valve. If it has vacuum, then it should actuate the arm on the valve.

 

Thanks for help on the blue I followed it to the 4x4. I checked the vacuum to the heater control valve and it is good the valve itself is brand new. I replaced the thermostat again to make sure I had put the right one in and I did. My next day I can I'm going to try back flushing the heater core again with clr.

 

Changed heater core now have heat. But broke my evaporated core. Did not know the evaporator core was actually in the housing under the dash. I knew I would have to unhook the ac. So now I have a spring project.