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  1. #1
    Active Member Two Rings
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    Heater Core Flush Question

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    I have a partially obstructed heater core. I removed the heater core hoses and flushed the core in both directions several times using Prestone radiator flush with boiling water. Some particulate crud comes out but man it's a slooowwww process.

    Iím wondering if I couldn't I use the old two-part radiator flush/neutralizer (oxalic acid/baking soda) on the heater core only? From what I can tell (based on a few photos Iíve seen on the Ďnet) the water tubes and the spiral mixer elements inside the tubes seem to be copper or brass or bronze. The fins are aluminum but they don't seem to come in contact with the coolant. I don't know about the ends of the core--I assume the copper/brass/bronze tubing is just bent into a 180 degree turn. Does anyone know if there is anything but copper/brass/bronze in the coolant flow path in the heater core?

    Since oxalic acid is stronger than the Prestone (citric acid, basically lemon juice) it should shorten the process. Any reason I shouldnít do this? I know this will likely be temporary but I donít have the time/money to replace the core now.

    I see people use CLR but when I read the label it says NOT to use it on copper, brass, aluminum and a whole host of other metals. So that gives me pause. At least oxalic acid was used as a radiator flush. Any thoughts?

  2. #2
    Active Member One Ring
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    Oct 14 2009
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    01 A4 Avant 1.8t Q 5sp, 90 toyota supra turbo, 94 civic hb gsr swap
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    Just take it to a radiator shop. It will cost you a couple bucks but, I've found the results are usually worth it. If you have the hoses off and they can just hook up to your heater core the cost should be minimal.

  3. #3
    Active Member Two Rings
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    Awww, but what's the fun of that? I'm always looking for a zero dollar fix. Since I have some old wood bleach (oxalic acid) and baking soda I thought I could do this for nothing! Any idea what chemical(s) they use for flushing?

  4. #4
    Veteran Member Four Rings BlackLock_A4's Avatar
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    Sep 14 2009
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    San Jose

    I used Drano and had 2 1L water bottles on each end to flush it out one way and back in - kinda get it movin around in there.
    2001 1.8TQM
    Speed doesn't kill. It's that damn sudden stop.

  5. #5
    Veteran Member Four Rings walky_talky20's Avatar
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    Jun 30 2008
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    Erie, Pennsylvania

    I've used CLR once on a Toyota Rav4 heater core. Filled and let it sit for like 30 minutes. Didn't get bubkis out of it. I actually saved the CLR for later because it came out just as it went in. I thought that stuff would be the bomb-diggity, but I wasn't impressed that time. I'd certainly try it again sometime, though.
    ^Don't listen to this guy, he's not even a mechanic.
    2001 Laser Red A4 1.8TQM, 5-Speed Swapped, APR 93, Splitter, electronic oil pressure gauge, B6 Sport 17's, ST Coilovers,
    Need to borrow a Cam Chain Tensioner Tool? $4 for shipping. PM me. *available*

  6. #6
    Senior Member Three Rings shokwav09's Avatar
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    Quote Originally Posted by walky_talky20 View Post
    I've used CLR once on a Toyota Rav4 heater core. Filled and let it sit for like 30 minutes. Didn't get bubkis out of it. I actually saved the CLR for later because it came out just as it went in. I thought that stuff would be the bomb-diggity, but I wasn't impressed that time. I'd certainly try it again sometime, though.
    that makes me wanna try oxy clean lol
    since i was a child, everytime i saw a lamborghini, it represented a mans resillience to oppose an imperfection. and i swear god blessed ferrucio and his honor to stand the trial of time through ingenuity. with pure speed

  7. #7
    Established Member Two Rings esee135's Avatar
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    Jun 21 2008
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    Canada

    CLR worked for me! I also flushed it with compressed air to drive out the fluids and CLR, then clear rinse with hot water, kept about 20 psi on it, bunch of crud came out

  8. #8
    Senior Member Three Rings shokwav09's Avatar
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    im curious what a micron filter inline could do for keeping it cleaner after a flush or replacment
    since i was a child, everytime i saw a lamborghini, it represented a mans resillience to oppose an imperfection. and i swear god blessed ferrucio and his honor to stand the trial of time through ingenuity. with pure speed

  9. #9
    Active Member Two Rings
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    I think a micron inline filter would clog much too quickly, but I understand where you're going. Grainger sells a clear plastic-bodied filter with a cap that unscrews to expose what looks like a stainless steel mesh basket. I can't find it right now but I'm thinking about tracking it down and splicing it into the supply hose for the heater core. The way I see it, the heater core is just a pre-filter for the radiator. So why not a pre-filter for the heater core?

  10. #10
    Senior Member Three Rings shokwav09's Avatar
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    Quote Originally Posted by BikerBill View Post
    I think a micron inline filter would clog much too quickly, but I understand where you're going. Grainger sells a clear plastic-bodied filter with a cap that unscrews to expose what looks like a stainless steel mesh basket. I can't find it right now but I'm thinking about tracking it down and splicing it into the supply hose for the heater core. The way I see it, the heater core is just a pre-filter for the radiator. So why not a pre-filter for the heater core?
    why not a prefilter for the prefilter? hmmm? smartguy.
    since i was a child, everytime i saw a lamborghini, it represented a mans resillience to oppose an imperfection. and i swear god blessed ferrucio and his honor to stand the trial of time through ingenuity. with pure speed

  11. #11
    Established Member Two Rings
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    Nov 12 2010
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    2002 a41.8tqm 92 audi 100s 93 audi 100s
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    Newark, DE

    ive been thinking about doing this my dad owns a construction company and he he said caterpillar makes a radiator flush that is the best stuff on the market for heavy equipment im thinkin about using this on my b6. I really want heat its getting old relying on heated seats.

  12. #12
    Active Member Two Rings
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    OK, so I did the two-part flush (oxalic acid/sodium carbonate) on just my heater core a few days ago. At that point I had already done about 5 flushes with Prestone Heavy Duty Flush and boiling water. The oxalic acid dislodged a lot more crud and, interestingly, the sodium carbonate neutralizer also released a bunch of really dark colored material. I followed that with a bunch of flush/backflush cycles with water and compressed air. I was still getting a little bit of particulate material but the water was very clear.

    It was 15 degrees here this morning and my car was toasty warm. I'm not sure the heater core is back to its original condition but it's working pretty well now. I don't know how long this will last but time will tell. I'll be happy if it stays like this through the winter.

    If you're thinking about trying this make sure to read GM bulletin #99-06-02-012D first. GM had a problem with their DEX-COOL coolant and recommended using this old two-part flush to get rid of the resulting rust and corrosion products. The neutralizer sodium carbonate (washing soda, same as pH increaser chemical used in pools) shouldn't be left in contact with aluminum for more than 10 minutes at boiling temperatures as it is mildly corrosive to aluminum. I still don't know if my heater core has copper tubing with aluminum fins or if the tubing is aluminum also. Clearly, you don't want to eat a hole in your heater core. As always, you do this at your own risk.

  13. #13
    Veteran Member Three Rings protocol_droid's Avatar
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    ^^So how did you physically do it? Use the Silicon tubing and add the solution into the core and then cap off the other end and let the solution sit for a few hours and then do the second part carbonate for a few hours and flush?

  14. #14
    Active Member Two Rings
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    I bought ten feet of clear vinyl tubing from the Homely Depot and cut it in two pieces. I don't recall the hose diameter but someone on one of the Audi forums has a write up with the correct diameter. If you have trouble slipping the tubing on the heater core pipes just dip the ends of the tubes in boiling water to soften them(probably not a big issue in CA but in colder climates vinyl gets very stiff). Suspend both tubes with a bungee cord from the hood latch mechanism so they don't fall down and spill chemicals on you or your car. Blow the coolant out of the heater core. Use a funnel and pour in the oxalic acid/boiling water solution until you can see liquid a few inches up both tubes from the point where they attach to the heater core pipes. I didn't cap either tube. I would periodically blow gently in one tube to move the solution back and forth a little. I let it sit for 20 - 30 minutes, blew it out with an air compressor (into a clean white catch pail so you can see what's coming out), put in some more hot mixture, agitate, blow it out. I did that about 4 times. Then boiling water and sodium carbonate for ten minutes. Don't leave the sodium carbonate in longer than that in case you having aluminum tubing in your heater core as it's mildly corrosive to aluminum. But I think that's why it's also effective at getting more crap out of the core.

    FWIW, the old two part cooling system flushes contained 9 ozs oxalic acid and 2 ozs sodium carbonate. The volume of the heater core is really small, maybe a cup and a half, so you don't need a lot of either chemical to do this. I used maybe a tablespoon or two of oxalic acid with that cup and a half of water, less for the sodium carbonate. You should wear goggles and use rubber or nitrile gloves just to be on the safe side. Oxalic acid is what makes parts of the rhubarb plant poisonous.

  15. #15
    Veteran Member Three Rings protocol_droid's Avatar
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    ^^ Nice with the bungie cord. I"ll be doing this over the weekend. I already purchased the tubing from home depot.

    BTW, you should be able to just use sodium bicarbonate (arm n hammer) to neutralize as oxalic acid as it isn't that strong an acid to need carbonate to neutralize... if people are looking for a cheaper alternative. I happen to have em both anyhow.
    Last edited by protocol_droid; 01-12-2011 at 09:42 AM.



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