360 [solved] Crankcase vent

I'm hoping that someone can help me understand the science behind this problem:

I currently have a Yoshifab crankcase vent plate. The left fitting is the oil drain and the right fitting is the PCV port.

I have my catch can set up like this to drain back to the sump. It also has the oil filler cap vented into it.

This setup works fine but I'm sick of oil entering the intake before the turbo, so in an attempt to reduce it, I installed another catch can that I had around. I just took the hose going to the intake before the turbo and re-routed it to the catch can first.

It's a simple catch can. No baffles, just an open can on the inside. But when I connect it inline to the PCV hose going to the intake manifold, it acts like a plug and I end up getting crazy amounts of crankcase pressure. To the point where the valve cover and oil cap leak profusely and I get smoke out of the exhaust after a short drive.

I fitted the old PCV/oil separator box and testes if that did the same thing and it does. I put one way valves inline with the hose going to and coming from the catch can and it still did the same thing.

I don't understand why this would cause extreme crankcase pressure?


  • Do you have the piping diagrams for the cans that you can post?

  • No I don't, exclude my setup out of the equation, I realis it just overcomplicates the question.

    The same excessive crankcase pressure happens with the factory setup. Instead of the large hose coming from the PCV/oil separator and going straight into the air intake pipe, it goes to a catch can first.

  • Alex, I can't speak for the second can unless there is a hidden valve in it that is closed or plumbed backwards.

    As for the first can, given that it is a separator and is subject to fluid mechanics, there are 3 main things that cause separator vessel issues.

    1. The separator is under sized for the vapour/liquid volume.

    2. The piping is incorrectly installed (unlikely in your case).

    3. The internals are missing or poorly designed (i.e. there isn't a coalescing mesh/pad for the droplets to adhere to).

    Without knowing the catch can and the air flow volumes it is difficult.

  • Thanks Ian,

    The first can works fine and doesn't cause any excessive crankcase pressure unless I connect it to the second can.

    The same is true with the factory oil separator/PCV. The second can Is just hollow. No baffles, no restrictions.

    Here's some pics of someone else who has done the same thing:

  • This is the hollow catch can it's going into.

  • @Roinik Hey Ian, as per point #3, the first can does have a baffle plate, but it doesn't seem enough to stop the oil vapour going to the intake before the turbo. I have since stuffed some steel wool in there and it appears to have reduced the oil vapour considerably. The steel wool wasnt present when I was having issues with the other can.

  • Do you run the tube in the block down to the oil pan still? Is it intact?

  • It has been shortened to about 1.5". Only used as a seal.

  • My catch can recommends a check valve in the oil return close to the engine if it is above the oil level of the sump. Could be something to try.

    Personally I'd give the second catch can a miss and if you haven't already, chuck a steel scrubber in the primary can.

  • It won't work properly as an oil return if it's cut short like that. Volvo designed it the way they did for a reason.

    The original long length is designed such that the end of it is below the oil level. Than way oil can freely run down it as nothing's trying to come back up it.

    When it's short and hence way above the oil level, gases from the crankcase are coming up it. Makes it real hard for oil droplets to flow the opposite way back to the sump.

  • edited September 13

    Thanks Sam, that's a good suggestion with the check valve.

    I've already put some steel wool into the top of my catch can. Seems to do a great job at preventing oil entering the intake.

  • There's a bit of info on TB about this. Many people cut it short without any issues and Stealthfti states that it only really goes to to sump to prevent oil foaming?

    I don't have any problems with oil draining back to the sump. The catch can doesnt fill up and drains fine.

    Anyway, the check valve makes sense and would eliminate and pressure entering the drain hose...but my issues are greater than that. I've just completed a compression test:

    Cyl#1: 100psi

    Cyl#2: 110psi

    Cyl#3: 100psi

    Cyl#4: 110psi

    B230et with 531 head. Initial compression test 1000kms after rebuild was 150psi on all cylinders.

    Explains my constant valve cover leak and excess crankcase pressure...

    I have a squirter block that I will rebuild and go from there. Back to the drawing board, I guess.

  • edited September 17

    Check valve made a big difference, thanks.

    Edit: just to clarify, it appeared made a big difference at reducing crankcase pressure...or at least helped expel the pressure out of the catch can.

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