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722.6 (C250 CDI) blowing ATF from breather...

Discussion in 'Engine, Drivetrain, Fuel and Exhaust' started by moj91, Sep 13, 2019.

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  1. moj91

    moj91 Senior Member

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    Hi All,

    Had a happy gearbox, now its unhappy...

    Working just fine, 115k miles..
    Had the gearbox oil/filter/plug changed by a good indie (as advised by an autobox specialist a while back, (preventative maintenance)).

    Drove to Cornwall for a week break, no issues, excellent changes etc.

    On the way back noticed a little dampness on the undertray... not sure why looked under the car but i did, didnt think much to it.
    Following day, noticeable smoke (ATF getting onto exhaust) and pretty significant leak.
    Checked level with dipstick, just above the bottom cold so added 100ml increments until back into correct range and drove maybe 10 miles gently to garage. All changes etc still perfect.

    Garage checked plug o ring again as we thought that was the leak, turns out not, its blowing fluid out of the breather on top of the box, and pretty significantly now.

    Id be quite surprised and disappointed if the gearbox has suffered a major failure internally, its been looked after, and only on 115k miles. ATF was previously changed at around 40k miles as was the consensus to do at the time.
    Our other cars with 722.6 box are all over 200k without issue, bar one which had a TC fail at 220k ish, and had not had any oil changes.

    Anyone know what could be going on?

    Thank you!
     
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  2. Jim2

    Jim2 Senior Member

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    Sometimes..they can do that if they are overfilled.. but I'd always check mine when the box is at 80c..I never do it when cold. So as a first reaction, that's what I would do, if only to eliminate the oil level as the cause. After you cleaned it up etc. did you notice if it did it again, or has it been Ok since?
     
  3. umblecumbuz

    umblecumbuz Senior Member

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    Sounds like an overfilled box. Does the Indie realize that you fill the box while the engine is running with the selector in Park, and the box at operating temperature?

    If the box was filled with the engine stopped, then it would be very much overfilled even though the dipstick will show the correct level.
     
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  4. alexanderfoti

    alexanderfoti MBO Forum Supporter Authorised Forum Supporter

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    Yes, the only reason they blow out the top is from being overfilled. Engine running and in park as above.
     
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  5. OP
    moj91

    moj91 Senior Member

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    Well this is the thing, it did possibly 700 miles before it started blowing the ATF out after the oil change.
    Both the indie and I have checked the levels with various correct dipsticks at cold and up to temperature, with the 'starting' levels correct it is till spewing out, yet the gearbox is operating otherwise normally.
     
  6. alexanderfoti

    alexanderfoti MBO Forum Supporter Authorised Forum Supporter

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    The oil expands significantly when warm, @70Deg. I would still suggest that the box is overfilled. Are you checking it with the correct dipstict?

    I only check it at 70Deg.

    upload_2019-9-16_10-37-26.png
     
  7. OP
    moj91

    moj91 Senior Member

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    We've been using the lazer dipstick set, with the correct dipstick as per their guide. The biggest alarm bell, i suppose, is that the car did maybe 700 miles before the ATF started noticeably spewing out/burning off the hot exhaust and smoking, but is still changing/engaging/accelerating smoothly. If it was overfilled from the initial oil change I would have expected it to spill out on the 30 mile drive home from the garage.

    I/we rechecked the level with the Lazer dipstick, inserted to the correct depth (also using another car, same gbox as reference, which showed correct levels at the depth inserted both cold and to temp) whilst the box was up to temp, and running in Park. Level by this point had again dropped to the bottom of the dipstick, so 100ml increments were added to bring the level up to the correct 80c marks, still whilst running in so that the added oil could warm up and expand. This just resulted in the added fluid being blown out again over a short drive. Having changed a 722.6 gearbox oil myself before, and being assured by the indie they had done countless ones in the past (I have no reason to distrust the indie in question) I am still hoping that it is somehow overfilled (hence it is still operating correctly) but I think we would have ascertained this by now - I fear an internal issue, hopefully not with the gearset, but maybe with the valves/valve body/my knowledge therein of these gearboxes thin on the ground haha! Maybe the float has jammed open? Who knows at this point.

    We will see, the car is still with the indie. I'll report back with the diagnostic/resolution once we have it, even if it is a 'tail between legs' answer... Any further input is certainly welcomed!
     
  8. umblecumbuz

    umblecumbuz Senior Member

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    Even after reading your last post - with the gearbox showing no other faults, and operating exactly as it should, I am still inclined to believe that overfilling is the cause of your problems.

    When many of the forum contributors on here change atf and filter in the 722.6 box, they save and measure the old fluid, and then replace the same volume with new. This gives an excellent datum, and minor fluid additions can then be made if needed with confidence.

    Your Indie should have measured the quantity of fluid that he put in. If so, he will tell you how much went in. If he just filled using the dipstick as a guide, this may also be a pointer to your problem.
     
    Last edited: Sep 17, 2019 at 6:43 PM
  9. ajlsl600

    ajlsl600 Senior Member

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    interesting. so how much do you put in pre start . as cant be good to have it running after draining with little/no oil in ? or are we working on the basis that not all the oil was extracted in the first place, just wondering ,and what a ballache they have made of this process.
     
  10. alexanderfoti

    alexanderfoti MBO Forum Supporter Authorised Forum Supporter

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    Sump drain
    Filter replacement
    Back together
    Put 3.8l in
    Start engine
    Check level at 80
     
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  11. bembo449

    bembo449 Senior Member

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    im still getting used to the box codes , is this a 5 or 7 speed ??
     
  12. alexanderfoti

    alexanderfoti MBO Forum Supporter Authorised Forum Supporter

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  13. bembo449

    bembo449 Senior Member

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    will my 170 have that box then Alex ?
     
  14. alexanderfoti

    alexanderfoti MBO Forum Supporter Authorised Forum Supporter

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    No, the a classes are different. They have a different type. (722.1)
     
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