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  1. #1
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    Cabin air filter central plenum cracked

    This one’s a bit weird. I have a ’99 528i, and when on a road trip awhile back, I came to a stop, and I began to smell the unmistakable smell of motor oil coming through the a/c vents, and just to amplify that sensation, a puff of smoke came through the center “fresh air” air registers! It became obvious that I was experiencing a valve cover gasket failure, but what would explain that puff of oil smoke? Besides why in the heck would the climate control’s carbon monoxide sensor/automatic recirculation mode NOT be tripped off by a big puff of burning oil right off of the exhaust manifold and instead make it’s way into the cabin?

    The answer is that while installing a new valve cover gasket today, I found that there is a good-sized crack or fissure right down the middle of the plenum that supports the two under-hood cabin air filters. No wonder I got a straight shot of burnt Castrol 10-60 synthetic to the schnozz that day! I’ll take a pic of it and post it tomorrow.

    At any rate, has anybody ever experience this cracked cabin filter central plenum issue? Is it DIY-able? Thanks to the Forum in advance.


  2. #2
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    Member No: 25422 larrychew is an unknown quantity at this point larrychew's Avatar
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    How about silicone caulk as a fix?


  3. #3
    Craig in Canada
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    some answers

    I have some answers/comments on your post....

    1/ The CO sensor is not "in" the HVAC system, it is clipped to the fan shroud at the front of the car under the upper rad hose on the driver's side of the rad. It's not going to be in the air path at the rear of the engine compartment.

    2/ Having disassembled the underhood area of the HVAC air path, and the interior portion (complete removal of the dash) to replace the blower fan on my dad's 2002 540i, I'm not sure that the air path works the way that you think it does. I can't be sure until I see your pictures. The ducts from the cabin filter boxes lead straight into a flap which closes when the car is off and when recirc is engaged. They also close partially depending on the road speed of the car to prevent pressure build up. These circular input ports directly feed into the two squirrel cages of the fan unit. There is no "central plenum" until after the blower underneath the dash. Even then, the notion of left/right airflow is preserved because of the split system.

    So whatever crack you see may not be a crack in any air plenum and doesn't need "fixing" other than cosmetics. Of course you clearly smelled oil. A common reason for this is that the rubber seals around the cabin air filters have crushed and are no longer sealing against the underside of the hood meaning air can directly enter the intakes from the underhood area. You can test with lipstick or chalk on the seals and close the hood.

    It's also common for the tabs at the firewall end of the intake tubes to break. Between this, possible improper installation and degrading seals here, you could also be taking on completely unfiltered air from the rear of the engine area.

    Another possibility is that your VC gaskets are leaking enough oil that your car simply smells. Everything could be working and sealed properly and a strong burning oil smell would still enter the system if that's what the air around the front of your car smells like. Note that if you aren't running activated carbon filters (and they aren't fresh) there is nothing about the HVAC intake system that would remove the smell anyways.


  4. #4
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    Re: some answers

    You're absolutely correct, I do suffer from a fairly basic ignorance of how the airflow is routed in the E39's HVAC system, and I suppose a cursory look around the engine compartment might have led me to make a few incorrect conclusions based on a hastily made and incorrect assumption regarding the non-existent "central plenum", so thanks for setting me straight. Still not exactly sure what I'm looking at in terms of the cracked plastic at the upper edge of my firewall, but it may just be an extension the "windshield frame cover" at the base of our windshields, that tends to crumble like Enron. I'm going to post a pic of it, it's rather unsightly, and I think I'd like to take care of it, regardless of whether the problem is more cosmetic than functional.

    I have an old Honda CRX for a daily driver, and have been driving the 528 only very infrequently, but whenever I felt compelled do drive it, the smell of oil when the car warmed up a bit served as a reminder to hit the recirc mode on the HVAC control, at which point the problem was mitigated virtually in toto. I'll be sure to pay close attention to the cabin filter box's installation, condition of the seals as well as for the connection of the ducts at the firewall. I appreciate your assistance with this problem.

  5. #5
    Craig in Canada
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    Re: some answers

    Whenever you can post some pics, please do. That will be very helpful to determine if your cracked piece is cosmetic or functional. Have you located the oil leak? Compared to V8s the I6s don't have a lot of valve cover gasket problems.

    You don't have quite the same engine as I do (I have a 1998 528 with the M52, you have the M52TU) but you'll be happy to know that a VC gasket replacement is pretty straightforward if that is what you need. It's all pretty easy to access on the I6.

    If you're handy you could also do the Beisan double-VANOS seal retrofit while you're in there :) Even if you don't, the whole procedure for VC gasket replacement is in the VANOS procedure from the Beisan site:

    http://www.beisansystems.com/procedu..._procedure.htm


  6. #6
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    Pic of crack(s)

    That underhood plastic resembles a weathered Italian Renaissance painting up close!
    The valve cover gasket R&R is my first project ever and now I feel more like a full-fledged member of Roadfly, since this forum is chock full of guys who aren't afraid to tackle anything. I had thought about the Beisan Vanos seal kit, but quickly dismissed it, but since I'm partially there, I'm giving very serious thought to pulling the Vanos. Hope the pics work!

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    Re: Pic of crack(s)

    Here's another pic:







  8. #8
    Craig in Canada
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    Re: Pic of crack(s)

    That plastic is pretty bad, but that area shouldn't be carrying any HVAC air to my knowledge.

    Check that the ducts are fully seated and that the seals are good where the ducts attach to the firewall (grey open-cell foam can be seen in your pic). It may help to use a silicone lubricant on the rubber parts during reassembly. Also check the black rubber seal around the filter boxes against the underside of the hood.

    Enjoy your wrenching :)

  9. #9
    Panos
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    correct. That should be replaced to protect >>

    the engine wire harness but no HVAC air is housed by that cracked piece. Its actually quite common for that to crack. I believe the updated part is more resistant to heat cycling.

    I agree with Craigs recommendations.

  10. #10
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    Re: correct. That should be replaced to protect >>

    Although I have the 540i mine looked just like yours. It only took me 30 minutes or less to replace and the part at BMW was only $35. It'll make a mess when you pull it out. Be sure to cover your cylinder head well before you start to remove it.

    David'06 BMW X5 4.8is Sapphire Black/Black mfg 10/2005 58k
    '01 BMW 540i Sport Jet Black/Black mfg 11/2000 90k
    '00 Jeep Wrangler Sport White/Tan cloth 82k
    '06 BMW Z3 Rider Green/Tan Seat (daughters push car)

  11. #11
    doctorgonzo
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    Luckily, David's right, not too pricey....

    ...for that piece. I guess it will be back to the parts counter in the a.m.

    By the way, (I just have to crow about this to somebody) out of nowhere, my local BMW parts counter has started selling me everything at a discount, I think the guy used the term "service cost". I paid $333 for a new secondary air pump and 95 bucks for the accompanying valve. Chalk it up to my nice-guy attitude and winning charm!

    Thanks for the help, I'll start a new thread once I start the VANOS seal replacement.

  12. #12
    gc
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    I for one am interested in how the plenum

    replacement goes. Mine is almost entirely gone and I've been delaying the repair. Pictures and a quick little write up would be most appreciated.
    Greg

  13. #13
    doctorgonzo
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    Re: I for one am interested in how the plenum

    I've never posted a DIY with pics or anything, primarily because I'm a horrible mechanic, but I'll attempt to post one for this piece. However, I'm having a hard time finding the part on realoem.com. The plastic at the base of the windshield is looking pretty ratty and I was able to navigate the menus and find that:

    RealOEM.com Home => Select another car => Vehicle trim => Exterior trim => Grille





    However, I don't think this part encompasses the piece that is disintegrating, or does it? If not, I can't seem to find the plastic piece in question (between the two cabin filter boxes). No biggie, I'm sure the man at my local BMW parts counter can help, but it'd be nice to see online before I buy.

  14. #14
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    This looks like it's the part you are looking for(

    Hard to tell from the pic, but it looks like it's correct.
    FYI, the part in your link above will require you to pull the wiper arms off to replace it. You might need a puller to get the arms off.

    http://www.realoem.com/bmw/showparts...51&fg=80&hl=20

  15. #15
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    I wouldn't have spled that in a million years! Thx


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