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Fixed Orifice PCV design and issues. #5271380 11/18/19 10:44 PM
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Talent_Keyhole Offline OP
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I am looking for theory and operation on fixed orifice PCV designs. My GM service manual provides little information on it. Interested in learning about the pro's and con's of this type of crankcase ventilation. My 2012 GM Ecotec Gen II 2.4l has a fixed orifice in the intake manifold/cylinder hear flange and fresh air feed from the air plenum with a basic separator. These engines are prone carbon build-up in the orifice and when the separator fills up and freezes, crankcase pressures rise and blow the rear seal. I am aware of the TSB on this problem. A vented breather cap with a check ball is not an option and just a bandaid.

I assume the fresh air side also serves to evacuate the crankcase at speeds and higher loads, as the fixed orifice will only flow so much volume at a given vacuum. I am seeing 20" of Hg at idle and drops to 5" and even less than 1" at moderate loads at highway speeds. The fixed orifice obviously will not remove the blow-by at the rate necessary so I assume the air flow through the air plenum makes up the difference in theory.

At these speeds and loads, during cold, high humidity (snow and rain) I see large amounts of moisture, oil and blow-by in the fresh air separator and the oil fill cap is heavily coated. During the same weather conditions, driving in the city, with much higher vacuum readings, the crankcase and separator stays clear, even in short trips, where the engine does not fully reach operating temps.

I have been searching the internet and forums for more detailed information on fixed orifice PCV theory. I see that it is favored over traditional variable PCV valve designs when it comes to ECU fuel management and reduction in oil consumption.

Any knowledge shared and links to more details would be appreciated.




Last edited by Talent_Keyhole; 11/18/19 10:50 PM.
Re: Fixed Orifice PCV design and issues. [Re: Talent_Keyhole] #5271409 11/18/19 11:30 PM
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clinebarger Online Content
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Both types of PCV valves can freeze....Some Fords have heated PCV valves.

GM may have a updated valve cover?


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Re: Fixed Orifice PCV design and issues. [Re: Talent_Keyhole] #5271419 11/18/19 11:52 PM
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oldhp Offline
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Why couldn't a person take the tube off the valve cover, put a hose on it with a breather filter on the end??? Yes it would be open to atmosphere but that's better than blowing the rear main seal, and it wouldn't freeze in the air box anymore either. Mount the breather filter "higher" than the valve cover, that way no water would collect inside the breather. IDK.......

Last edited by oldhp; 11/18/19 11:54 PM.

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Re: Fixed Orifice PCV design and issues. [Re: oldhp] #5271420 11/18/19 11:59 PM
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Talent_Keyhole Offline OP
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Originally Posted by oldhp
Why couldn't a person take the tube off the valve cover, put a hose on it with a breather filter on the end??? Yes it would be open to atmosphere but that's better than blowing the rear main seal, and it wouldn't freeze in the air box anymore either. Mount the breather filter "higher" than the valve cover, that way no water would collect inside the breather. IDK.......


Air flows in both directions on fresh air side. Unmetered air causing a lean condition and the breather will fill with emulsified oil/moisture and freeze. The stock separator is located 6-8" above the camshshaft cover.

Re: Fixed Orifice PCV design and issues. [Re: clinebarger] #5271423 11/19/19 12:05 AM
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Talent_Keyhole Offline OP
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Originally Posted by clinebarger
Both types of PCV valves can freeze....Some Fords have heated PCV valves.

GM may have a updated valve cover?



The fixed orifice in the head/intake flange heats up fairly quickly and it is fed by multiple baffles in the valve cover. Freezing is not a problem with this one, only carbon build-up. The fresh air/separator easily freezes and takes quite awhile to thaw out.

I checked, no updates. Ford and others have updated their covers to address PCV and oil consumption issues.

Re: Fixed Orifice PCV design and issues. [Re: Talent_Keyhole] #5271566 11/19/19 07:38 AM
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In cases like this I like to add an external catch can ahead of the PVC orifice. This allows most of the hot crankcase vapor to cool and condense, before the unburned fuel and other vapors continue to the orifice, Across the orifice there will be a pressure drop, so the air will cool more and more stuff will condense. Since most of the stuff ends up in the catch can, the problem is reduced. with regular draining of the catch can, frozen moisture in there will not restrict flow.

We do not have harsh winters in southern Missouri anymore, so maybe this is a poor solution for where you are. I try to locate catch can in a cool area under the hood'. In Iowa this may not work.

Rod

Re: Fixed Orifice PCV design and issues. [Re: Talent_Keyhole] #5271638 11/19/19 09:30 AM
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Originally Posted by Talent_Keyhole
..Freezing is not a problem with this one, only carbon build-up. .


That's been my understanding of them as well. I know the Ecotec 2.2 in my 2007 Cobalt was setup like this and I * think* the inline 6 in my 2005 Trailblazer was as well.


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Re: Fixed Orifice PCV design and issues. [Re: Talent_Keyhole] #5271652 11/19/19 09:39 AM
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SHOZ Offline
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A few on my Gen Coupe boards swear by direct vent to atmosphere and basically eliminate the PCV valve.


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Re: Fixed Orifice PCV design and issues. [Re: ragtoplvr] #5271803 11/19/19 12:32 PM
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Originally Posted by ragtoplvr
In cases like this I like to add an external catch can ahead of the PVC orifice. This allows most of the hot crankcase vapor to cool and condense, before the unburned fuel and other vapors continue to the orifice, Across the orifice there will be a pressure drop, so the air will cool more and more stuff will condense. Since most of the stuff ends up in the catch can, the problem is reduced. with regular draining of the catch can, frozen moisture in there will not restrict flow.

We do not have harsh winters in southern Missouri anymore, so maybe this is a poor solution for where you are. I try to locate catch can in a cool area under the hood'. In Iowa this may not work.

Rod


Owners are not able to add a catch can or separator ahead of the orifice because it is buried in the intake. There are baffles in the camshaft cover coming from the lower crankcase, going to the orifice, and from the fresh air port on the cover. GM did not design a drain on the separator inside the air plenum. The owner has to remove the entire housing and turn it over. A catch can be installed between the air plenum and camshaft port.

Has anyone come across a theory and operation information on this variation of a PCV system utilizing a fixed orifice and fresh air camshaft port that serves both as fresh air feed and a vent to the low pressure side of the throttle body. This concept has been used in many vehicles in the past years.

Re: Fixed Orifice PCV design and issues. [Re: Talent_Keyhole] #5272214 11/19/19 10:02 PM
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Originally Posted by Talent_Keyhole
Originally Posted by oldhp
Why couldn't a person take the tube off the valve cover, put a hose on it with a breather filter on the end??? Yes it would be open to atmosphere but that's better than blowing the rear main seal, and it wouldn't freeze in the air box anymore either. Mount the breather filter "higher" than the valve cover, that way no water would collect inside the breather. IDK.......


Air flows in both directions on fresh air side. Unmetered air causing a lean condition and the breather will fill with emulsified oil/moisture and freeze. The stock separator is located 6-8" above the camshshaft cover.


But if the filter is open to atmosphere the air can go in/out of the filter. As I said mount the filter higher than the VC, any oil, moister etc will drain back into the head. I wouldn't think the air flow is unmetered because the only opening that sucks vacuum is the small hole in the head going to the intake manifold. The open filter doesn't go to the intake manifold, just the inside of the head area. Right??? Should work to me.


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Re: Fixed Orifice PCV design and issues. [Re: oldhp] #5275419 11/23/19 12:05 PM
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Originally Posted by oldhp
Originally Posted by Talent_Keyhole
Originally Posted by oldhp
Why couldn't a person take the tube off the valve cover, put a hose on it with a breather filter on the end??? Yes it would be open to atmosphere but that's better than blowing the rear main seal, and it wouldn't freeze in the air box anymore either. Mount the breather filter "higher" than the valve cover, that way no water would collect inside the breather. IDK.......


Air flows in both directions on fresh air side. Unmetered air causing a lean condition and the breather will fill with emulsified oil/moisture and freeze. The stock separator is located 6-8" above the camshshaft cover.


But if the filter is open to atmosphere the air can go in/out of the filter. As I said mount the filter higher than the VC, any oil, moister etc will drain back into the head. I wouldn't think the air flow is unmetered because the only opening that sucks vacuum is the small hole in the head going to the intake manifold. The open filter doesn't go to the intake manifold, just the inside of the head area. Right??? Should work to me.


And where does the small orifice in intake/head get its fresh air......from the breather which is open to atmosphere and not metered in your suggestion. This will cause 10-15 point fuel trim increase. The stock reservoir and oil fill cap is above the camshaft cover, height makes no difference. Others have tried a CFM vented cap with check ball and it also collects emulsified oil/water and freezes. The manufacturer warns not to use it unless the engine is at full operating temps.

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