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Heat blows cold when slow or stopped

C

Christoph006

NAXJA Forum User
Location
Louisville, KY
Just bought a 91 Cherokee. The heat gets hot but I notice it gets cool or lukewarm when I slow to a crawling speed or am stopped. If I switch the blower off then back on after a minute or so of driving to Heat or Defrost it’ll be hot again. Previous owner replaced radiator, water pump, thermostat and heater core.

Is this a vacuum issue? If so how would I go about finding it?


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cruiser54 said:
Ditch the heater valve while you're at it.

Totally unnecessary and there is no downside to eliminating it.
Click to expand...



I’m unfamiliar with the heater valve. Where is this?

I checked the radiator at the cap. No fluid to that point but there’s what looks like a return or vacuum port to the left of the cap in the picture that’s uncapped allowing air into the system. Doesn’t that feed into the overflow?
0e39e8f0d74baf53e5eb994307bf3c2a.jpg



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Heater control valve is inline with the heater hoses near the firewall. I got rid of mine a long time ago. Just run some new hoses. Ands there should be a hose on that barb to the overflow.

That coolant is nasty. Flush it and the heater core and change the thermostat.
 
Christoph006 said:
I’m unfamiliar with the heater valve. Where is this?
Click to expand...

Up near the firewall. Connect the hoses together with a double barb (or better just replace the hoses with new heater hose, and cap off the valve line. Flush the heater core back-n-forth a few times to get all the crap out. Looking at that picture the whole system could use a flush.

8211245-1__ra_p.jpg
 
ccb60abd91c6b3f14e481fa95fa1349f.jpg
da9cb2a1669570ce20c8589a7f030772.jpg


Flushed the system today and replaced all hoses. I saw a guy on YouTube that showed the hoses for the 97+ XJs were molded perfectly to fit because they don’t use the heater control valve and just have to cut and splice the 5/8 and 3/4 hoses to go into the heater core. Cleans up the engine bay of some clutter and I routed the overflow line along the firewall. Heat is very hot now and isn’t blowing when I turn the hvac to off.

Thanks for the help everyone! Much appreciated!


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trippled said:
More than likely it's a bad thermostat. Seen that a lot. Won't hurt to flush the heater core at the same time.
Click to expand...

Pretty easy to change the thermostat or do I need to remove a few components out of the way prior to getting access to thermostat housing?
 
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