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  1. #1

    Clearing a clogged heater core

    The heater in my '89 MJ has never worked in the 2 years I've owned it. Electrically its fine [cept no low speed on the blower].

    The cores about 80% clogged. 4 flushes the old fashioned way using over-the-counter prestone crap has brought it from totally clogged to 'some' flow thru it so on cold days I have a little heat but on 0 days - not enuf.

    NOT THE STAT - NOT AIR IN THE SYSTEM! It IS the core. I'm no fool I know how to troubleshoot a core. I also know what a MAJOR PITA it is to replace the core unless I was rich and had some other monkeys do the wrench spinnin for me and I just put it on my plastic. I dont go that route.

    ANYWAY.....

    I am going to backflush it with a garden hose to see if theres any gobs of junk that may get kicked out when flow is reversed but before I do that I thought I'd add something to loosen up the matter clogging it.

    I have heard 50-50 arguments that liquid drano works great but may damage rubber components. Since the only rubber is external to the firewall in the form of heater hose to/from the thermo housing - I can live with replacing them. I also heard that vinegar can help clean it out too. I have no access to the muriatic or other nice acids the shops use so its going to be the store bought stuff.

    Anyone have any experience in this that can add some insight? I've had plenty of guessing from many who have no idea what they're talking about and I've had my fill of it so some real insight would be a nice change.

  2. #2
    bowtied1's Avatar
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    It will probably start to leak if you can actually back flush it. It usually takes boiling them to really clean them. I would recommend that you replace it with a new one.
    Blackish 91 YJ coiled blah blah blah.

  3. #3
    MDert's Avatar
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    When I flush a system I drain the coolant, back flush it, then fill it with water and drive it til it gets hot as piss with the heater on. Then drain it all out. I usually repeat this a few times depending on the severity.
    If it's too far gone itll have to be removed and rodded out or replaced.
    '94 YJ- 5.3, 4L60E, NP241, D60, 14B, 107", 42"s
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  4. #4
    VortecCJ's Avatar
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    I've always had good luck disconnecting the heater hoses and using compressed air.
    1986 CJ7 60/14, 6.0, th350, Atlas
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    -ASE Certified Master Technician-

  5. #5
    How hard are the heater cores in the MJ/XJ to replace? That was one thing I never got to do on my XJ. That said, I would just replace the core and be done with it. I wouldn't want something to come loose and clog the rest of the system later on. Or worse, have the core start leaking in the interior.
    94 YJ 0cyl BDS 3.5", 5/8 Booms, SYE
    Waggy 44s, 1" BL, ARBs, 35" MTRs

  6. #6
    Ive done the compressed air fix too with good results.
    www.totalfab.net

  7. #7
    Quote Originally Posted by lilgreenjeepyj View Post
    How hard are the heater cores in the MJ/XJ to replace? That was one thing I never got to do on my XJ. That said, I would just replace the core and be done with it. I wouldn't want something to come loose and clog the rest of the system later on. Or worse, have the core start leaking in the interior.
    I spent an hour trying to remove a core from a boneyard MJ and finally gave up. It was that much BS. The entire dash has to come out, plus if you add A/C to the equation it becomes even more of a pain. Removal is out of the question. I'd rather deal with it as is than mess with that.

    Guess I'll stay the original plan and hook a hose to the outlet and blast it backwards after soaking it in vinegar overnight. Although its an '89 and should have a closed system, the idiots who owned it prior put a closed system on it, Overflow res, rad cap, even a vac powered flow control valve even though no XJ/MJ I ever saw had anything but a blend box setup. Its a total Frankenstein truck trust me...!
    '91 XJ 4.0L HO 4.5"
    '89 MJ 4.0L 5"
    '95 ZJ 4.0L HO

  8. #8

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    R & R'd the heater core for a buddy with a 89 It was a major pain in the ass, Stupid band that holds it up you have to cut, its not right after you remove and replace. It had a A/C and it SUCKED, had to remove the hoses and pull the entire dash out, one side to the other. I won't do another heater core unless its in a 1979-83 yota (its easy 4 bolts and the hoses and you can slip it right out without taking the dash out, the dash on the yoter is simple though).
    One flopped yota

  9. #9
    The problem with the early XJ/MJ was that about half of the coolant went through the plastic bottle instead of the core. If you convert to a newer rad and use the newer heater hoses and valve you will be amazed at how much more heat you will have from your heater.
    America, land without a president as of 1-20-09 R.I.P.

  10. #10
    satan's Avatar
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    I've had some practice on the Renix years X/MJ cores -- 6 total -- 6 suck.

    However, try just a soft back flush (3/4" garden hose) with the thing fully disconnected you'll be amazed at how much will just come out without adding any cleaning or etching additives... (use only a little pressure - and let a steady reverse-flow do all the work)

    The heater flow control valve only shuts when the Hot/Cold slide is full cold -- and this doesn't fully stop the flow (the heater circuit holds about 18% of the total coolant volume on a 4.0 Renix, so it's gotta keep moving) The intent of the flow control is just to allow the A/C a little break to not have to work against the full heater core. The valve in 87-90 should look like this:


    I had one really sucked/clogged core in a 90 -- ended up leaving the hose on it overnight (frustration actually - no real pressure from the spigot - and no flow when I went to bed) - I woke to find rust and crud washing down the driveway and the best part is that I got no leaks (without jacking with the core).

    I totally hate the R&R of that core - plastic and rivets and a lot of time IMHO... Good luck to ya. ( all that said - I used a bike pump, an old tire pressure gauge and a few lengths of 3/4/ cheap-ass garden hose to fab-up a leak-down tester to help assure that I had no leaks in the core - handy thing to do before you're "Back in service" to find out that you've created a "stinky fog generator" in your cab...)
    Last edited by satan; February 23rd, 2010 at 08:40 AM.

  11. #11
    Resident Gorilla
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    I've had luck unclogging a heater core with a vacuum system that some of the dealers have. It is used to bleed the system by pulling vacuum on it and then allowing a rush of liquid into the cooling system. It will pull the hoses down flat before you open the valve to let fluid in. We did this and it pushed a slug of gelled antifreeze out of the core that was blocking it. If you can get access to one of these systems, it only take a couple of minutes and it worth a try before pulling a core.

    All the system is, is a valve that connects to the radiator cap opening with a hook up for compressed air (which generates the vacuum on the system) and a connection for antifreeze.

    Here is the valve system that I was thinking of:
    http://www.amazon.com/Airlift-Coolin.../dp/B000O3IGI6
    Last edited by Camp; February 23rd, 2010 at 09:02 AM.
    "If we can prevent the government from wasting the labors of the people, under the pretence of taking care of them, they must become happy." Thomas Jefferson

  12. #12
    Quote Originally Posted by satan View Post
    I've had some practice on the Renix years X/MJ cores -- 6 total -- 6 suck.

    However, try just a soft back flush (3/4" garden hose) with the thing fully disconnected you'll be amazed at how much will just come out without adding any cleaning or etching additives... (use only a little pressure - and let a steady reverse-flow do all the work)

    The heater flow control valve only shuts when the Hot/Cold slide is full cold -- and this doesn't fully stop the flow (the heater circuit holds about 18% of the total coolant volume on a 4.0 Renix, so it's gotta keep moving) The intent of the flow control is just to allow the A/C a little break to not have to work against the full heater core. The valve in 87-90 should look like this:


    I had one really sucked/clogged core in a 90 -- ended up leaving the hose on it overnight (frustration actually - no real pressure from the spigot - and no flow when I went to bed) - I woke to find rust and crud washing down the driveway and the best part is that I got no leaks (without jacking with the core).

    I totally hate the R&R of that core - plastic and rivets and a lot of time IMHO... Good luck to ya. ( all that said - I used a bike pump, an old tire pressure gauge and a few lengths of 3/4/ cheap-ass garden hose to fab-up a leak-down tester to help assure that I had no leaks in the core - handy thing to do before you're "Back in service" to find out that you've created a "stinky fog generator" in your cab...)
    I actually just removed that valve. The third leg was capped off [usually goes to the stomach shaped res] plus the valve didnt seem to open very far. I eliminated it completely, capped off the vac hose [where'd it come from? The truck shouldnt have one]. Everything still works the same without it.

    Theres no way I'm pulling the core, I keep saying that. This afternoon I am doing the backflush route and will report the results in the next day or so. I can bring home an air tank and fab up a quik-disconnect with a blow gun. If I blow the stupid core up - I asked for it by being cheap.

  13. #13
    satan's Avatar
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    The vacuum line is off the HVAC controls (really) only sucks when your temp is set fully to cold.

    I'm a bit confused by (I'm thinking you ment to say the POs put and open system in it?):
    Quote Originally Posted by MJs4ever View Post
    ... Although its an '89 and should have a closed system, the idiots who owned it prior put a closed system on it, Overflow res, rad cap, even a vac powered flow control valve even though no XJ/MJ I ever saw had anything but a blend box setup. Its a total Frankenstein truck trust me...!
    and how the post says " not supposed to have the heater flow valve ' ... If you have A/C (or it did) they all should'a had some valve (even the 2.5s) - all 3 of the 4.0 MJs I had had A/C and the valve

    I suppose it's not too relevant though -- more about getting your core flow set to rights.

    The vacuum idea sounds interesting - and could help things along without exposing the core to a lot of pressure (15psi from a garden hose into a 17"hg vacuum would give you ~23 delta-psi on the core trying to flow without getting all wild and over-pressuring anything! It'll be interesting to see what all comes outta there and if you can get the flow up -- I'd definitley stay wth reverse flow and stedlily increase the flow rate as it clears -- just be careful of the available pressures ...

  14. #14
    If you want some heavy duty stuff try this: http://www.fleetguard.com/pdfs/produ...LI33024-GB.pdf

    Most Cummins dealers should carry it. This is not the cheap, worthles crap you buy at Checker or Napa. If you leave it in the cooling system too long it WILL damage stuff due to the high level of acidity, at least with the Restore Plus.
    The regular Restore is not an acidic flush but it sounds like you may need the hd stuff.

  15. #15
    Ya Satan I meant to say someone put an OPEN system on it but I would've been AMAZED if anyone caught that - You did..Congrats! ..

    I will start with the less dangerous like vinegar and a garden hose backflush, cept my H2O pressure is 110psi and thats constant cracked or wide open if anyone knows their physics they know thats true only the volume changes with more flow so I will be easy on the damn thing. If the easy approach fails I'll keep stepping it up with he harsher chems - anything beats pullin the core. Dang truck cost me less than the cost of that!

  16. #16
    Lilly Bear ColoradoXJ's Avatar
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    I've heard that Allen at Hillside 4x4 loves to work on heater cores. It's his specialty.



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  17. #17
    satan's Avatar
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    Quote Originally Posted by MJs4ever View Post
    ... - anything beats pullin the core. Dang truck cost me less than the cost of that!
    -- ZACTLY !!! i'm hopin' it works-out well for ya...

    ... I've aloway though that a plasma cut into the core area from the firewall side would be a fun thing to do... so if'n ya git to that point -- I'm in! (will bring the cutter & beer; cannot be held responsible for collateral damages)

  18. #18
    Quote Originally Posted by satan View Post
    -- ZACTLY !!! i'm hopin' it works-out well for ya...

    ... I've aloway though that a plasma cut into the core area from the firewall side would be a fun thing to do... so if'n ya git to that point -- I'm in! (will bring the cutter & beer; cannot be held responsible for collateral damages)
    Yer into mass destruction, ain't ya?? I got a gun you'd LOVE!

    Well heres the 411....

    I disco'd the heater hoses, blew the coolant out of the core via my lungs [got one hell of a head rush doing that], poured vinegar into the core & left it for about 30 min. I found a pair of washing machine inlet/outlet hoses in my Smithsonian attic and used them to mate the house 110psi H2O supply to the Prestone tee for flushing. Then I blasted water thru the core backwards for about 30 min.

    TONS of crap came out! Eventually, clear water flowed. I hooked up the heater hoses BACKWARDS since it seemed to flow better that direction, then I topped off the coolant, went for a drive, and ta DAA!!! Massive heat came from the blower.
    MY TRUCK HAS HEAT!!!

    I may be driving to AZ this weekend and just knowing that I wont freeze my jingle bells off is comfort enuf! Dang I wish I'd done this a year ago!
    Last edited by MJs4ever; February 24th, 2010 at 05:59 PM. Reason: typo

  19. #19

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    Quote Originally Posted by MJs4ever View Post
    I may be driving to AZ this weekend and just knowing that I wont freeze my jingle bells off is comfort enuf! Dang I wish I'd done this a year ago!
    Shouldnt you be worried about the AC

  20. #20
    satan's Avatar
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    Boy, I hope the heat will shut off for ya now (in AZ) -- it'd suck to have the twig and berries all boiled in pools of man-sweat 'cause yer heater output can't be toned-down

    Good to hear - you may wanna coolant flush it all after runnin for a while to keep the newly freed residual crud from migrating into somewhere else!

  21. #21

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    Drive a few days and flush the entire system!
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