How long to cool down before draining coolant?

Discussion in '80-Series Tech' started by roncruiser, Mar 12, 2007.

  1. roncruiser

    roncruiser

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    Location:
    San Diego
    I typically run a slight variant of this procedure for flushing my radiator:

    http://forum.ih8mud.com/showpost.php?p=1614118&postcount=4

    One thing I've always wondered is how long to let the vehicle cool down before draining and filling again.

    I know its not a good thing to drain a hot engine and refill with (cool) fluid. There will be the temperature shock to the block just from filling with cool liquid amongst many things that could go wrong. Not to mention HG problems. A friend of mine cracked the block on a VW by draining a hot engine and filling with cool liquid.

    I typically wait an hour before draining the coolant from a hot engine. Sometimes I think an hour is not enough. Some of the posts I have read on radiator flushing don't mention a cooling time. So, I'm assuming they don't wait at all.

    How long is a good time to allow a hot engine to cool before a drain and fill?
  2. IdahoDoug

    IdahoDoug

    Messages:
    8,782
    Agree longer is better, but I think by 15 minutes the block's hot spots have evened out and the block/coolant temp is about equal. At that point, the entire mass is simply cooling down together very slowly as there's very little circulation of coolant (random convection movement). So OK to drain.

    On filling to flush, I think you have to know this step is where a thermal shock can be delivered (vs draining - not such an issue). So, fill it slowly so the water going in can be warmed as the level rises in the block. Put it on a pencil thin stream, stuff the hose in the coolant hose and relax until it starts coming out the radiator. Never blast it in wide open.

    For the final fill - same thing. Take a full 60 seconds to pour a gallon of distilled in.

    It's the refill that can deliver the shock. I would not expect damage to be as easily done as some think, but it's common sense.

    DougM
  3. NaterGator

    NaterGator On Gilligan's Island SILVER Star

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    I stick a couple of box fans around the hood area just to get some airflow around the engine bay and cool the whole area off as much as possible... it is still slow and relatively even, but alot quicker. An hour with 2 fans going near the hood would be plenty, I'd say.
  4. Grench

    Grench SILVER Star

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    Omaha, NE
    2x on 15 minutes. I filled/flushed ours 10+ times each before I was happy with it.
  5. roncruiser

    roncruiser

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    943
    Location:
    San Diego

    Thanks. I agree with you. Just making sure, again, that there are no gremlins unbeknownst to me waiting to bite once I began the procedure.
  6. Driley

    Driley

    Messages:
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    Location:
    Bend, Oregon
    Quick question: Is it necessary to use distilled water for the flush procedure tarbe describes? Can you just use regular tap water for the flushes then use distilled w/ antifreeze for the final fill?

    Thanks,

    Riley
  7. Got Cruiser?

    Got Cruiser?

    Messages:
    199
    Riley,

    I just got back from buying a radiator from Performance Radiator for my 97. I spoke to the tech advisor and he said it's okay to use tap water to flush out your cooling system. He showed me how much damage can tap water do to your cooling system. Definetely use distilled water for final application. My son has a 95 and he and i are going to do a complete cooling system change this weekend.

    Quick question: Is it necessary to use distilled water for the flush procedure tarbe describes? Can you just use regular tap water for the flushes then use distilled w/ antifreeze for the final fill?

    Thanks,

    Riley
  8. Grench

    Grench SILVER Star

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    2,662
    Location:
    Omaha, NE
    Distilled is cheap all things relative. I did a double distilled flush before 2 gallons of Toyota red and topped off with distilled. Overkill? Yes sir.
  9. DIHRD9R

    DIHRD9R

    Messages:
    26
    Location:
    Arizona
    yea i thought an hour was the general cool-down time....being in arizona though, an extra half-hour never hurt
  10. Grench

    Grench SILVER Star

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    Location:
    Omaha, NE
    If it's cool enough that you can stand to put your fingers on the block plug to pull it, then its cool enough. (Yeah, that test could hurt.)

    Seriously, there isn't any reason to do this with the coolant so hot it hurts. You don't have to wait for it to be stone cold either.
  11. medtro

    medtro

    Messages:
    4,252
    I wonder how long a shop would wait when doing this job.

  12. roncruiser

    roncruiser

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    Location:
    San Diego
    Good question. I've wondered the same thing.
  13. Yotato

    Yotato

    Messages:
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    Location:
    Roseville, CA (Sacramento)
    I just had my cooling system flushed this weekend and they didn't let it cool at all. They claimed that the coolant going in was warmed. I doubt it, but no issues after the flush and change.

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