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Thread: Question on oil breather cap

  1. #11
    Senior Member DMC5180's Avatar
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    Jonathan,

    Without getting into Professorial detail, The Internal combustion engine is a big Air pump that requires a fuel/air combustion process to keep it moving.

    As the piston moves down during the intake stroke, it creates a LOW pressure condition which draws in (atmospheric pressure) based on the differential of HIGH to LOW. Within the intake system there is a Throttle plate that causes a measured restriction in airflow. Upstream of the throttle is the Filtered Intake or air inlet. Downstream of the plates is the Upper deck (manifold pressure side). When the pressure is LOWER on the Manifold side of the throttle plates it creates a vacuum that can be utilized for auxiliary purposes ( all vacuum ports) are downstream of the plates. When the throttle plates are closed you are at maximum differential, creating strong vacuum. Engine RPM plays into this too. Think high RPM deceleration. Attached too the manifold side of the intake is the emissions system, evaporative fuel vapor and Crankcase vent (Blow by recirculation) and a vacuum tank storage system with a CHECK VALVE too smooth out the transitional process between open throttle and closed.

    Note: back in the OLD days before the invention of electric windshield wipers, they were vacuum powered. I always thought it was funny when driving my Uncles 1936 Dodge pick-up to a car show and having to drive through rain showers during the trip. Because there was no storage tank system to draw from, when you step on the gas pedal to accelerate or climb a hill and the one arm wiper stops moving due to loss of vacuum (throttle plates open). let off the gas to coast and it's back in action.
    Last edited by DMC5180; 06-23-2013 at 12:23 PM.
    DENNIS

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  2. #12
    Admn Retired Ron's Avatar
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    Jonathan,
    First, yes the air filter housing is what is called the breather.

    You have the direction of flow wrong. W
    hile the engine is running, the cold start tube is under vacuum from the intake manifold, created by the pistons on their downward stroke (exhaust valve shut, intake valve open...its port is connected to the intake manifold..). Also connected to cold start tube is the cold start valve (figures huh? nothing goes through it but fuel), a line from the charcoal canister, and a line from the oil filler cap (which also has a line to the breather, another source of vacuum. I'll get to that in a second). If the throttle plates are closed, crankcase vapors (simply combustion that gets past the piston rings, or blow-by) will flow to the filler cap, to the line going to the cold start tube, to the intake manifold (just like fuel when cranking cold) and burnt. Meanwhile vapors in the charcoal canister are being held. When the throttle plates are opened, a vacuum line connected to a valve in the charcoal canister opens, allowing them flow to the cold start tube, to the intake manifold and be burnt (higher RPM makes for cleaner emissions). [If you are wondering why the signal line didn't have vacuum on it, it is because it picks vacuum up on the other side of the throttle plates (ie upstream of the air flow)]. Here is where the other vacuum source at the oil filler cap comes in (relies on what Dennis posted, which should make all of this much clearer.) When you open the plates, blow-by at the filler cap will flow through the line going to the breather instead because it will have more vacuum than the line going to the cold start tube (Technically- It can handle more volume because of its size...it couldgo both ways...and the vacuum isn't necessarily greater, but almost always is when off idle...and a bunch of crap you don't want to know LOL).



  3. #13
    Admn Retired Ron's Avatar
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    Quote Originally Posted by DMC5180 View Post
    Note: back in the OLD days before the invention of electric windshield wipers, they were vacuum powered. I always thought it was funny when driving my Uncles 1936 Dodge pick-up to a car show and having to drive through rain showers during the trip. Because there was no storage tank system to draw from, when you step on the gas pedal to accelerate or climb a hill and the one arm wiper stops moving due to loss of vacuum (throttle plates open). let off the gas to coast and it's back in action.
    First, good explanation!

    I've seen that before but it wasn't THAT long ago, on a school bus. It is weird. You probably remember Ford was plagued in the late 70s-80s with the AC and defrost dropping out when they took a load like climbing a steep hill (due to using vacuum activated/powered actuators) too?

  4. #14
    Concise Member Jonathan's Avatar
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    Hey, cool guys! Thanks to both Ron and Dennis. I recall taking a combustion engines course in 4th year, but one of the lecture hours was bright and early on Friday morning and I'm thinking I might have missed a few too many of those time slots by hitting the snooze button, lol. I think I still have the text book lying around and I might just browse through it!

    So... with the mention of the high RPM deceleration... would this tie in in some way to how the brake booster functions? I know it's not along the lines of an engine brake you might see on a big rig or something, but stronger vacuum if being utilized in the braking system may make it more effective at higher RPMs? Does that make sense?
    One damn minute Admiral...


  5. #15
    My friends think I'm nuts jawn101's Avatar
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    Wow, this has turned into a very informative thread.

    Just a note on the premier cap, I just put on about 80 miles and didn't notice any difference, despite the much tighter seal. I do want to check out my dipstick though now, the seal around the actual stick has never seated into the tube, at all. Is the seal movable on the stick or is it fixed?
    Jon
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  6. #16
    Senior Member Dangermouse's Avatar
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    Quote Originally Posted by jawn101 View Post
    Wow, this has turned into a very informative thread. ?
    Hasn't it just!

    I need to re-read it tomorrow with a diagram .
    Dermot
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  7. #17
    Admn Retired Ron's Avatar
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    It's in the manual, but broken into two diagrams.....

  8. #18
    Senior Member Dangermouse's Avatar
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    Quote Originally Posted by Farrar View Post
    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.
    Farrar, you don't happen to know the size you got that worked?
    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"

  9. #19
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    Quote Originally Posted by Dangermouse View Post
    Farrar, you don't happen to know the size you got that worked?
    Sorry for the epic thread resurrection.

    I've just installed a 41mm ID 5mm thickness o-ring I ordered online from Simply Bearings in the UK I'm sure there is an equivalent in the US. They work out to be pence each if you order a bag full.

    It's a much better seal than my old one, which had gone hard, but could possibly be improved if you can find 6mm thickness with the same ID. I think 5mm is far more common though.

  10. #20
    Senior Member Rich's Avatar
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    Quote Originally Posted by jawn101 View Post
    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.
    The vendors seem to have them. I recently replaced my original with one from Grady - now the original cap sits tight enough that it stays in place. Didn't expect any other changes, just a proper look and fit. The old o-ring wasn't tight like it used to be and it had a crack in it.

    Nice that you got a better-looking Premier cap that does what you want, Jon. Creative, that.
    March '81, 5-speed, black interior

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