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01xjltd 01-10-2009 08:32 PM

99 cherokee heater core replacement tips / low heater output
My 99 cherokee has low heat output when full hot is selected, checked the blend door motor which seems to be working. I changed the thermostat, and radiator and backflushed the heater core which was plugged with rad sealer, but when flushing from both tubes of the heater core with both air and water, core opened up and began to pass fluid to what seemed to be normal. But still no help in correcting problem. Any Ideas ? Any inside tips on replacing the heater core ? Running engine and monitoring temp {which was slow to come up to 195} on scanner, leads me to think stat might be ok, and blend door as well.........Thoughts ?

SaharaMan 01-21-2009 12:50 AM

I'm having a similar problem. Try this out, I did it today to see what was wrong with mine....

01xjltd 01-21-2009 05:37 AM

Thanks for the heads up ! I suspect that the control / temp selector is bad because I took my infrared thermometer and checked the water temp going into the core and comming back out. Also I flushed the core out and checked flow against a brand new one {its replacement} and flow rate was the same. I then checked to see that the blend door motor was moving and it was. Temp comming out of the vent was about 86 deg, so its my opinion that the control head / temp selector is not selecting / driving blend door full hot. Ill let you know how it goes after i get a new temp controller installed.

kadetklapp 01-21-2009 09:35 AM

Good luck. I have had flow thru my core, hot coolant in and out, what I believed to be adequate flow, and even when I cut a hole in the heater box and moved the blend door by hand I get ice cold air.

01xjltd 01-21-2009 03:50 PM

Well That tidbit puts a new kink in the idea of the temp control. Looks like i will be going to the scrapyard to pull the whole friggen setup out and disassemble it to see how this could manifest. This might sound like something like some kinda foam / sponge the blend door closes against might have dried out and cold air might be leaking around it. one thing is certain I will get a fix for this, as low heat is not acceptable

01xjltd 08-05-2009 05:13 PM

low heater output
i have found so far that this appears to be related to the defroster door not having adequate vacume supply or the actuator is bad or the actuator attachment is broken off. I will soon disassemble the unit and report findings

dictusm 09-12-2009 11:55 AM

Have you solved this issue? I am having the same problem. I replaced radiator and hoses, flushed heater core -seems to have plenty of flow. Took blend door motor off, blend door opens and closes fine. Motor operates fine. Temperature changes from cold to outside temperature when running ac, if I turn the dial from all the way cold to all the way hot. Hoses going into and out of heater core are both hot. No coolant leaks, plenty of coolant in the system. Water pump seems to be working fine, thermostat is good, temperature of engine rises and maintains temperature. I'm at a loss. Getting to the point of ripping apart the dash and replacing the core even though I don't think that is the problem.

01xjltd 09-30-2009 08:22 PM

heater core replacement tips
i have done some investagating and found the foam liner that connects the defroster duct outlet to the heaterbox on top missing as well as everything on the heater assembly otherwise working as intended. i suspect the core is partially blocked due to moderate heat after several miles.:(

edduyan 03-16-2011 11:45 AM

I am having the same problem with heat. If I leave the fan on low, I am able to maintain some heat coming through but once I turn the fan full blast, I start getting cold air. New radiator, thermostat, full coolant. Help...

AZ Jeff 03-16-2011 04:43 PM


Originally Posted by edduyan (Post 11170343)
I am having the same problem with heat. If I leave the fan on low, I am able to maintain some heat coming through but once I turn the fan full blast, I start getting cold air. New radiator, thermostat, full coolant. Help...

Plugged heater core is a likely suspect.

dictusm 03-16-2011 05:45 PM

Unplug the core
My heater did exactly the same thing. To fix it I removed both hoses going into the heater core and connected them to hoses one to water one to drain (after removing coolant of course). Flush the core for a few minutes then switch the hoses. Do it many many times to make sure all the junk is out of there. Good Luck.

edduyan 03-17-2011 10:46 AM

Thanks, I will do that this weekend.

01zerone 01-04-2012 05:53 PM

i had the exact same problem and like some of you guys, flushing out the heater core fixed it. and it's the easiest thing to do to start with, before you get into removing a thermostat and other things.

dmatt1116 06-26-2013 10:26 PM

Had the same thing happen to me. Sometimes you have to flush the radiator 2 or 3 times. Just flush it and go. If it persists check your thermostat. Flushing the heater core can be risky. You could upset it and be replacing it prematurely. :(

cpdr26 06-30-2013 09:53 PM

I've got a 99 XJ and just had all the problems you mentioned. No matter what I would do and check; it was never good heat. What finally cured it was getting a garden hose and blowing all the crud out of the core and about medium pressure, nothing crazy high (previous owner neglected cooling system)

Part two was this lil doo-dad.

I'll admit.... I thought it was kind of gimmicky but at that point it was January and I was so tired of being cold I said to hell with it. This is the one thing that has worked really awesome for me! There was so much residual crap in the system it kept making its way back to the core and clogging it. Its completely reusable and i've cleaned it about 6 times since installing it, and haven't had to since then. When popping the hood I can look right at it and see if its clean or dirty. Fluid is clean and green and heat is very much there.

Good luck!

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