Flush or change trans fluid

Discussion in 'Engine & Drivetrain' started by Bxcliff, Dec 6, 2018.

  1. Bxcliff

    Bxcliff Member

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    I've had my transmission rebuilt some time ago in my 2003 GMC Yukon Denali AWD and I'm changing both my differential and transfer case fluids. I also am planning to replace the transmission fluid filter so here is my question..

    Should I just change my fluid that comes out the pan or should I get it all flushed out with the machine.

    The fluid isn't all that bad, it's still sort of red but mostly brown, but it does look like it needs changing

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  2. Scottydoggs

    Scottydoggs Full Access Member

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    how many miles since the rebuild? if its brown looking only dump the pan, do the filter and refill it.

    a full flush is not always a good idea when the fluid is burnt up, its all thats making it work, change it all and it can die.
     
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  3. Bxcliff

    Bxcliff Member

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    I would say about 30 or 50k miles ago, so would it be best just to change the fluid and filter? It doesn't look burnt up at all just brown with a tint of red.

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  4. Dan Acosta

    Dan Acosta Full Access Member

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    If you decide to flush, Dont use the machine pressure feed style - let the pump do the work, so you dont force contaminate through the transmission, (At least that is the consensus). I used a 3/8 OD poly tube into the cooler return line and it fit perfectly. Was much easier than I thought it would be and didnt make a big mess. You will probably go through an extra gallon of fluid, but compared to what a shop charges it is minimal. (My 2¢)

    I used the method in this video:
     
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  5. Bxcliff

    Bxcliff Member

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    My friend at my job uses his machine that flushes atf out school buses and he also used It on my dad's 08 suburban. I dont know much about it but he hooked up both trans cooler lines to the feed and return and the fluid was being pushed out and new fluid being added.

    Is that what you're referring by it being pressure fed? We only did that because those trucks dont have a drain bolt but mine does. Would it be best to just change the fluid a few times while changing the filter instead?

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  6. SnowDrifter

    SnowDrifter Full Access Member

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    Drop pan, change filter, drain and fee pan a few times with a few mins if runtime in between.

    I don't know capacities off the top of my head, but you can calculate amount of new fluid in the system by:

    (1-((total capacity - fluid drained)/total capacity))^n *100%.

    Broken down, this is a ratio problem where n is the number of drain and fills of the pan.

    Any of the home style flush methods do essentially this but with the added risk of running your oil pump dry. But hey it saves the hassle of putting a vacuum tube down the dipstick hole?
     
  7. Bxcliff

    Bxcliff Member

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    Cool, thanks man I'll keep this in mind

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  8. Dan Acosta

    Dan Acosta Full Access Member

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    Pretty sure that is the pressure style - where the fluid is exchanged by an external pump. Usually those are ok for vehicles that are regularly maintained. Most transmissions in non fleet vehicles dont get changed as often as they should. You are going to hear a couple dozen methods of how to do this. If you were attempting to exchange all of the fluid, or most of it you are going to need to do the method I linked or the pump style. The 4l80e holds a lot of fluid, I think it is around 14qts or 3.5 Gallons. I probably used an extra gallon at least "flushing" it. Dropping the pan to remove that much fluid would take forever and never touch the fluid in the converter.
     
  9. Bxcliff

    Bxcliff Member

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    I dont want to risk using the machine to flush out the fluid, I dont think my dad serviced it at all after the rebuild.

    But about that video, is it safe to do that? My truck doesnt take a whole lot of quarts in the entire trans and I'm scared I might run it with no fluid for a moment.

    Also how much fluid does that remove from the entire system? In other words would I have to do it a second time possibly?

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  10. Dan Acosta

    Dan Acosta Full Access Member

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    Yes, it is safe. You dont want to run out of fluid of course, but you arent going run it dry if you shut it down when you see some bubbles. If you have the 4L80E you do have a lot of fluid in there like I said it is somewhere around 14 qts or 3.5 Gallons. There is a lot more in there than what is in the pan. The converter alone holds a few quarts. There is enough time from when you see some bubbles to shut it down yourself if you are working alone. If you feel better have someone sitting inside and you can have them shut it when you tell them (Or just get longer tubes). I actually added fluid as it was running to replenish what was pumping out - which is why I used a little more fluid. Empty the pan as the video shows ( by pumping or draining) change your filter, then top off before starting again. Then let it pump out and refill until you see a color change. I was a little reluctant, but once I did it I would do it again without thinking about it.
     

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