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Thread: Question on oil breather cap

  1. #1
    My friends think I'm nuts jawn101's Avatar
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    Question on oil breather cap

    Just throwing this out there as a general curiosity. I was at the junkyard with Clint today and picked an oil breather cap off an 88 Premier. The cap makes a much tighter seal in the oil fill neck, and fits the hoses to the airbox and intake manifold perfectly (I already have it installed on my car)

    The question is, this cap doesn't have the weird filter media inside. It's just an empty space. What's that media really for, anyway? Does it make a lick of difference if it's not present? This cap looks way nicer and fits much tighter so my preference would be to keep it, but if there's a reason not to I won't....
    Jon
    1981 DMC-12 #02100. July 1981. 5-speed, black, grooved w/flap.
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  2. #2
    Senior Member Dangermouse's Avatar
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    Funny, I was just wondering the same thing about how tight this should be.

    Mines pretty loose and I was gong to get a new O ring to see if that improves it, but I wanted to make a trip to the scrapyard anyway.
    Dermot
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  3. #3
    My friends think I'm nuts jawn101's Avatar
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    Quote Originally Posted by Dangermouse View Post
    Funny, I was just wondering the same thing about how tight this should be.

    Mines pretty loose and I was gong to get a new O ring to see if that improves it, but I wanted to make a trip to the scrapyard anyway.
    That's exactly why I hate the OEM one, but we haven't had much luck finding O rings that fit it! They are a weird, huge size.
    Jon
    1981 DMC-12 #02100. July 1981. 5-speed, black, grooved w/flap.
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  4. #4
    Voice For Hire Farrar's Avatar
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    I had a hard time finding a proper O-ring even at my local Ace hardware store which has lots of oddball stuff around. In the end I got one from John Hervey piggybacked onto another parts order. I though perhaps I had a vacuum leak there with an ill-fitting O-ring, but I didn't notice any change in running when I got one that sealed better. Go figure.
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  5. #5
    Senior Member Kenny_Z's Avatar
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    There's no vacuum in that area. It's just a spot for the engine to breathe. The only thing you have to worry about with an ill fitting cap is debris and dirt getting into the valves and cam.
    Red
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  6. #6
    Admins Never Retire Ron's Avatar
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    Actually it is not supposed to breathe...it is a closed crankcase system under vacuum. Check out where the two hoses go- breather AND cold start tube.
    (Something to consider when setting CO...)

    The media is there to filter the crankcase vapors...helps oil vapors condense and drip back into the crankcase...

  7. #7
    Senior Member Jonathan's Avatar
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    Interesting.

    Ron, I was looking at Dermot's other thread where this was being mentioned too and had a couple questions.

    - So if the vacuum is being pulled, is it being pulled from say the charcoal canister, through the tube that connects it to the cold start valve and then from the other port on the cold start valve and then to the one connection on the oil filler cap? So the direction of flow (if you will) would be from filler cap, through the tube to the CSV, from the other port on the CSV to one of the connections on the charcoal canister?

    - With that pulling direction in mind, what about the other connection on the filler cap, the one which goes up to the air filter housing? Is there air/vacuum being pulled towards the filler cap from this air filter housing as well? Part of that question is related to whether there is a proper installed direction for the oil filler cap? Not where it sits in the hole of course, but whether it matters which tube you have on which end? I am thinking in terms of the way a check valve often has an arrow shown on the side of it saying which direction the flow is. Wasn't sure if that applied to the filler cap or not. The second part to that is trying to understand the significance of the air filter housing (is this what is called the breather, btw?) connection and why it is there? Mostly because I think it was a photo or two of someone with a modified and cosmetically nice version of the air filter housing, but I don't think it had this tube going from the corner of it over to the filler cap. I don't understand the purpose of it so can't say whether it is necessary or not.

    - The odd thing is that the vacuum routing diagram doesn't show this tube going from the oil filler cap to the air filter housing. It might be left off for simplicity or clarity or perhaps because it isn't needed. Not sure myself, so that's why I ask.

    - And final question... what is it that generates the vacuum? If you want to produce pressure, such as moving water for example, you use a pump. I often work with centrifugal pumps in our water treatment systems. We also use vacuum pumps to do something very similar but instead of pushing water, they pull vacuum (air). We would use them on a "vacuum degasifier" type technology. They don't have the exact same centrifugal impeller like you would see on the water forwarding pump, but they often have a rotary lobe type design that accomplishes the vacuum pulling. What pulls vacuum in our engines? Is it this rotary lobe type idea of the crankshaft rotating and how the chamber in there is designed?

    Thanks for the lessons professor
    One damn minute Admiral...


  8. #8
    My friends think I'm nuts jawn101's Avatar
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    Quote Originally Posted by Jonathan View Post
    ...Part of that question is related to whether there is a proper installed direction for the oil filler cap? Not where it sits in the hole of course, but whether it matters which tube you have on which end?
    This much of your question is an easy one, the tubes are two completely different sizes And I'm also pretty darn sure there's no valving inside the cap at all. The one off the Premier motor is just a totally void space inside and I doubt the one for the D will be any different, except for the goopy, nasty filter media.

    As for the rest of your questions... I dunno

    I did notice the inside of the Premier cap was caked with oil residue which I cleaned out. It obviously served the same function as the D's cap, collecting and condensing vapors back into the motor.
    Jon
    1981 DMC-12 #02100. July 1981. 5-speed, black, grooved w/flap.
    restoration log, April 2012 to present
    restoration log, March 2011 to April 2012
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  9. #9
    Senior Member Dangermouse's Avatar
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    Goopy?

    Mine just looks like a ball of black wire string.
    Dermot
    VIN 2743, B/A, Frame 2227, engine 2320

    I don't always drive cars, but when I do, I prefer DeLoreans

    http://www.will-to-live.org

    No-one is to stone anyone, even, and I want to make this absolutely clear, even if they do say "carburetor"

  10. #10
    My friends think I'm nuts jawn101's Avatar
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    Quote Originally Posted by Dangermouse View Post
    Goopy?

    Mine just looks like a ball of black wire string.
    That's a good description of the media itself, but it's all coated in a film of oil.
    Jon
    1981 DMC-12 #02100. July 1981. 5-speed, black, grooved w/flap.
    restoration log, April 2012 to present
    restoration log, March 2011 to April 2012
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