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Thread: PCV system mythes and testing

  1. #1
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    Default PCV system mythes and testing

    As these cars get up in miles, more and more people need to do their PCV system. I've recently gone into quite a bit of detail on the PCV system and like to share some info and dispel any mythes. This is my working understanding. Please chime in if anyone has different understanding.

    In addition, I like to propose a more definitive PCV test. I've collected 2 samples using this test and like to get more. So perhaps people can try this simple test and report back.

    So onto the PCV design and function.

    Blowby Pressure

    Without going into much detail. There will be slight gasses leaking past the piston rings during its operation. This naturally increases the pressure in the crankcase and need to be exit somewhere so PCV system is basically the air channel design to vent this pressure. If PCV system becomes restricted (even partially), pressure will increase. Its same principle as if you put your thumb over a garden hose to create high pressure flow.

    Intake Manifold Vacuum on Turbo Cars

    IM is a great source of vacuum. Pistons draws in air on the intake cycle with throttle body plate being the air flow limiter. This produces a lot of vacuum and is used for PCV, EVAP, turbo CBV (compression bypass valve), and brake booster. However, the IM vacuum goes away and turns pressurized on turbo charged cars under boost. This means all things that use this vacuum source need an alternative solution under boost conditions.

    The easiest way to see this vacuum/pressure response in IM is to tap the IM at the CBV vacuum port. T this port and run a hose into into the cabin into a vacuum/pressure gauge. The vacuum/pressure dial basically responds to the accelerator. Applying accel moves the dial from vacuum to neutral to pressure.

    PCV's Idle Circuit

    When IM has good vacuum, PCV system uses this source to vent the blowby.

    PCV's Boost Circuit

    When IM is pressurized, PCV system needs alternative air channel to get rid of the blowby pressure. PCV's boost circuit basically creates a vent to front (air intake side) of the turbo to vent it. I've come to realize this is NOT a high vacuum source like IM because while turbo sucks in air, the air filter, air box and its air hose from the grill isn't much of a restriction. Thus the vacuum source to the boost circuit is relatively low. It is more of a venting port than say a vacuum source.

    What this means is you don't get negative PCV pressure during the entire drive cycle. A healthy PCV system should yield a little bit of positive pressure and this is okay! Many people believe it should always be vacuum and I think this is myth.

    There has also been observations Volvo's OEM sized boost circuit channel may not be sufficient as the engine wear gets up in age and require greater flow rate to vent the higher bypass.

    Diagram

    Now onto a diagram to explain this complicated things



    IM vacuum source is a 2mm air channel between the 2 #10 washers sandwiched by the #11 banjo bolt (towards the front of the car in this diagram). Here are 2 close up pics. First one has a line running through this channel to ensure its clear. 2nd pic shows the hose pulled off and the 2mm channel is inside of that barbed connection.

    Over the years, Volvo tweaked the PCV system. On the earlier P2s, the original Banjo bolt has an internal open air channel while later models includes a check ball. I think the reason for the check ball is to prevent the pressure from entering the boost circuit when IM is pressurized. But I suppose the boost circuit is sized large enough to vent both the crank case pressure and IM pressure under boost on older cars.




    Boost circuit is basically an aluminum pipe that runs inside of #8 and exit at clamp #13. Its clamped to the PTC nipple (more on this later) On a recent 2000 V70XC I worked on, this aluminum pipe is a relatively narrow 3/8" ID.

    Blowby gasses flows out of the block at the port clamped by #3 clamp and on the head at clamp #7. They both go into the oil trap (#1). These gasses has oily vapor in them and need to be filtered out and returned back to the engine oil while the pressurized air and various gasoline consumption byproduct gets sent back into the engine to be consumed again. The filtered out oil drains back into the oil pan at #4. Here is a picture of the oil trap internals, its design to route blowby gasses around some turns and have the oil vapor collect on the walls and drip down into the drain port.



    There is a complex drain channel inside the block and oil pan (At least 2 or 3 90 degree turns) and can clog/narrow the drain channel there. Dropping the oil pan is necessary to clear out this drain channel. I blew some air into the drain port and heard oil bubbling inside to ensure it was clear. Another member did the same and heard oil bubbling but decided to pull the pan and found partial restrictions. So you'll have to decide what you are comfortable with and how much effort to put in. Here are some pictures of this drain channel

    Lower red circle is the where the oil trap returns the oil.



    Long rectangular drain channel inside the block (this one is completely blocked)



    Another rectangular channel inside the pan. It eventually gets to the bottom and I believe makes another turn towards an upward facing exit.



    There are other lines in this PCV diagram and they are for coolant flow. Volvo thought it was a good idea to warm the PCV air to reduce condensation and therefore clogging. I'd imagine it quite a bit of over design complexity. This whole area is tucked under the intake manifold, it gets hot quick down there

    Test

    The simple glove test (wrap a latex glove over the oil filler hole) basically check the idle circuits vacuum. Revving the engine in park doesn't use the boost circult. You have to put the car in D, step on the brakes and gas it up to 2k+ RPM to enable the boost circuit. Needless to say you can't do this very long as it puts stress on the torque converter. Certainly not long enough to get a feel of how the boost circuit works in operation.

    A much better test is to take out the oil dip stick, route a long rubber/silicon tubing (5mm ID silicon tube works well) from oil dip stick port (jam the tubing in there to create a good seal) into the cabin with a latex glove rubber band to the end. You can't lock down the hood as hood seals would crimp the hose and also have to route the hose to avoid crimping. Now go for a drive, you can see PCV operation under idle, boost, and cruise.

    In the 2 cars I've seen this setup on with a healthy PCV system, glove definitely inflates under boost/cruise but its clear its not very high pressure. At times, it seems to have a mind of its own (maybe changing in relation to CVVT?) and doesn't exactly match the accelerator. Anyway, I consulted with a Volvo tech (cattlecar here), he said inflation is okay, just can't be high pressure.

    One way to make sure there is no high pressure is to swap the glove for a typical vacuum/pressure gauge. You definitely don't want to see the needle move much at all. Nothing even close to 1/2 PSI.

    Anyway, this makes sense, boost circuit is mostly a vent, not a vacuum port. A little positive pressure is to be expected. Would be great if others can run this test and we can collectively get more statistics.

    Increasing the boost circuit channel flow

    I've read/done resizing the boost circuit channel (using a 5/8" ID heater hose rather than Volvo's stock 3/8" ID pipe) on a 2000XC and definitely vents better (glove inflates less in above test). Another person drilled the PTC nipple's round restrictor hole bigger and also got less inflation.

    Anyways, hope this helps understanding this circuit and we get more stats on the above PCV test methodology. You might also ask what happens to EVAP, brake booster and turbo CBV when IM is pressurized? More complexity and I'll skip that here
    Last edited by howardc64; 02-09-2019 at 11:09 AM.
    Past Volvos : 01 V70 T5, 01/02 V70XC, 02 V70 NA, 00 V70XC
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  2. #2
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    Best is to go to your Volvo dealer, see if they will lend you the Manometer.
    Remove dipstick, turn off AC (important), plug the gauge into the dipstick hole, read the pressure.
    Gauge looks like a tire gauge, just seal it into the dipstick hole and read.
    If it moves a bit to the left it has some pressure. If it doesn't move or jigs over to the right, you have no negative pressure.
    If it pegs the gauge to the bottom, come back in 75K miles because it's as good as it gets!

    All this takes 30 seconds
    All emails please use: jrl1194 (at) aol.com

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    Quote Originally Posted by JRL View Post
    Best is to go to your Volvo dealer, see if they will lend you the Manometer.
    Remove dipstick, turn off AC (important), plug the gauge into the dipstick hole, read the pressure.
    Gauge looks like a tire gauge, just seal it into the dipstick hole and read.
    If it moves a bit to the left it has some pressure. If it doesn't move or jigs over to the right, you have no negative pressure.
    If it pegs the gauge to the bottom, come back in 75K miles because it's as good as it gets!

    All this takes 30 seconds
    Manometer is definitely the best quantitative measurement of the relatively minute flow rate. It will get an accurate reading on the idle circuit. For boost circuit, one would need to do the put in D, foot on brake, stump on gas trick and get a brief measurement. Kind of a shame to do such a brief and not very thorough reading while having such a high accuracy low pressure gauge.

    I'd be curious what dealer techs to other than a few second measurement on the boost circuit. Seems like regardless of instrument (manometer or rubber banded latex glove), its desirable to put it through a short test drive cycle to get an accurate insight on boost circuit health.
    Past Volvos : 01 V70 T5, 01/02 V70XC, 02 V70 NA, 00 V70XC
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  4. #4
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    Anyway, this makes sense, boost circuit is mostly a vent, not a vacuum port. A little positive pressure is to be expected. Would be great if others can run this test and we can collectively get more statistics.
    I've ran this test since our last PCV discussion and I have come to realize that the boost circuit is a vent rather than a vacuum source. The vent will also equalize crankcase pressure by not allowing too much negative pressure in the crankcase when the IM is in vaccum.

    My observations with the glove test:
    1. At idle - Crankcase should have slight negative pressure. If it blows up then it's a plugged idle circuit. If if trys to swallow the glove then it's a plugged boost circuit
    2. Brake Torque - With the glove still on the oil filler brake torque to approx 2k rpm. From my new understanding. If the glove blows up lightly than that's ok, pressure is being vented. If the glove blows up quickly or sustains pressure then it a clogged boost circuit.

    Now how much pressure is acceptable on load? LloydDobler on SS is trying to diagnose a popped cam seal. He suspects a clogged PCV vent and has rigged up a pressure gauge to an oil filler cap. At full load in 2nd gear (MT) he has registered 2psi.

    http://forums.swedespeed.com/showthr...ect-01-V70-T5M

    Myself, a slightly inflated glove on load is OK, but I would think that any positive pressure that can register on a gauge is not acceptable.
    Last edited by Antherzoll; 11-16-2015 at 07:33 AM.
    2005 XC70 Crystal Green | Hilton | 16T | Bad Swede | 130k miles

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    Quote Originally Posted by Antherzoll View Post
    I've ran this test since our last PCV discussion and I have come to realize that the boost circuit is a vent than a vacuum source. The vent will also equalize crankcase pressure by not allowing too much negative pressure in the crankcase when the IM is in vaccum.

    My observations with the glove test:
    1. At idle - Crankcase should have slight negative pressure. If it blows up than it's a plugged idle circuit. If if trys to swallow the glove than it's a plugged boost circuit
    2. Brake Torque - With the glove still on the oil filler brake torque to approx 2k rpm. From my new understanding. If the glove blows up lightly than that's ok, pressure is being vented. If the glove blows up quickly or sustains pressure than it a clogged boost circuit.

    Now how much pressure is acceptable on load? LloydDobler on SS is trying to diagnose a popped cam seal. He suspects a clogged PCV vent and has rigged up a pressure gauge to an oil filler cap. At ma load in 2nd gear (MT) he has registered 2psi.

    http://forums.swedespeed.com/showthr...ect-01-V70-T5M

    Myself, a slightly inflated glove on load is OK, but any positive pressure that can register on a gauge is not acceptable.
    Thanks Antheroll, this is exactly same as my understanding. When I asked cattlecar, he said positive pressure okay in boost circuit but definitely nothing over 0.5 PSI. So while the glove will inflate fully due to its small volume, it definitely should not be inflating instantly and stiffly stretched which would indicate a higher PSI.

    BTW, another quick test is to replace the glove in the cabin with a vacuum gauge. When I did this, couldn't register anything (gauge can't detect anything much lower than 1 PSI). This is a good trick to see the engine's subtle pressure change and to make sure nothing significant. I'll edit and note this in the original posting with the test procedure
    Past Volvos : 01 V70 T5, 01/02 V70XC, 02 V70 NA, 00 V70XC
    Current EV/Hybrid : 13 Tesla S85, 11 Gen3 Prius
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  6. #6
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    Quote Originally Posted by howardc64 View Post
    Thanks Antheroll, this is exactly same as my understanding. When I asked cattlecar, he said positive pressure okay in boost circuit but definitely nothing over 0.5 PSI. So while the glove will inflate fully due to its small volume, it definitely should not be inflating instantly and stiffly stretched which would indicate a higher PSI.

    BTW, another quick test is to replace the glove in the cabin with a vacuum gauge. When I did this, couldn't register anything (gauge can't detect anything much lower than 1 PSI). This is a good trick to see the engine's subtle pressure change and to make sure nothing significant. I'll edit and note this in the original posting with the test procedure
    This is a timely post Howard. Thank you.
    Our V70XC is at 165k. Would Mityvac setup a good alternate gauge for boost reading inside the cabin?
    I intend to include servicing oil pan too.
    2001 V70XC - Nautic Blue/ 2-tone Sand/ Premium/ Instant MPG 12 - 63/ Pirelli P4 12/32" - 740/ Duracell 48/ MOBIL-3309/ Pennzoil EURO 5W-40/ Zimmerman Z-coated/ Akebono/ 190K-mi/ Great Lakes

  7. #7
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    Quote Originally Posted by Xheart View Post
    Would Mityvac setup a good alternate gauge for boost reading inside the cabin?
    You want a vacuum+pressure gauge. I think the fancier Mityvac has that so should work if you got one that does more than vacuum.
    Past Volvos : 01 V70 T5, 01/02 V70XC, 02 V70 NA, 00 V70XC
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    Just a question I have been pondering the past few weeks, what are the reasons for there being two vent ports on these engines. The one on the top that is over the valve train and the one on the side of the block; Obviously more efficient to have the top and bottom both vented, but wouldn't the bottom and top of the engine all be connected by oil passage ways etc. What I'm really getting at, is if the bottom passageways to the flame trap are all clogged would simply bypassing the oil trap and venting the top valve cover provide some relief of pressure for the entire engine?

    I have a buddy with a S60R who is supremely clogged 4 months afters completely replacing the PCV system, he blows out the oil cap and his dipstick explodes oil all over the place. I told him to leave his dipstick sticking out a bit so the pressure doesn't build up; until I have time to fix it properly. I was wondering if I could simply run a hose from the valve cover to a catch can for the time being and avoid blown seals etc or would I also have to tap into the vent near the bottom side of the motor? He does have vacuum in the motor at idle.

    Thanks,
    Jesse
    2006 XC70 340,000KM, Hilton Tune, BadSwede Lift, Big Burly tires

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    Quote Originally Posted by IVIUSTANG View Post
    Just a question I have been pondering the past few weeks, what are the reasons for there being two vent ports on these engines. The one on the top that is over the valve train and the one on the side of the block; Obviously more efficient to have the top and bottom both vented
    This would be my guess. I recently came across an engine design that had an external tube between head and block to equalize PCV pressure between them. Can't recall which engine ATM.
    Past Volvos : 01 V70 T5, 01/02 V70XC, 02 V70 NA, 00 V70XC
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  10. #10
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    I've been running without a dipstick for about 6 months now to relieve the internal pressure in my clogged engine. Interesting discussion about various parts of the engine - it would seem that if blow-by is the cause, then venting the crank-case (as the dipstick does) should be enough.

    I ran a tube to a catch-can for a while, but found I was catching mostly condensed water, so I gave that up and have found only a very slight oilyness around the front of the engine after several high-mileage months.
    '04 XC70 (petrol/auto), Nautic Blue / Graphite, Premium, Touring, Boosters, Xenia Wheels w/235x60 Nokian WR's (wife's kid-hauler)
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