99 cherokee heater core replacement tips / low heater output

01xjltd · 01-10-2009, 08:32 PM

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....

http://www.jeepforum.com/forum/f13/automatic-temperature-zone-control-system-board-diagnostics-106446/

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

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

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

Quote:

Originally Posted by edduyan

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

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.