Testing the Flow of Engine Crankcase Vapors to the Air Cleaner

Having better understood the purposes of recycling Engine Crankcase Vapors through the

Air Cleaner, I wanted  to examine the transfer of Vapors from the Engine Crankcase  to the Air Cleaner.

There is a Professional Gauge for the purpose.  Not at my disposal, however.

David Brown adds to the discussion:

The experimentation on the Air Filter/Crankcase Breather question should provide some useful insights. I hope that you will keep me posted on it. The Air Filter should be pulling the air from the crankcase. I think that you will find some flutter at the outlet to the Breather Hose and you should perhaps be able to feel and  maybe measure the vacuum at the Air Filter inlet. Really “loose”, worn engines may produce more crankcase pressure than the Air Filter produces vacuum. I suppose that it feeds into the engine in either case but the more crankcase pressure there is, the easier it will carry oil droplets all the way to the Air Filter. This is bad for the engine–oily crankcase oil getting into the combustion chamber reduces octane and promotes destructive detonation. This crankcase pressure comes mostly from poor Piston Ring sealing.  I have a gauge that goes on the breather to measure this crankcase pressure. I used it on the Dyno when breaking in newly built engines and one can actually “see” the Piston Rings seating as the crankcase pressure drops. This is why a proper break-in procedure is so important. Worn Valve Guides can also add to the problem. Pressure up the Exhaust Valve Guides can add to the problem and pressure down the Intake Guides can deliver oil and air directly into the Combustion Chamber worsening the detonation problem. Try your testing both with a cold engine and a fully warmed up engine and you may see a big difference between them.

In order to test both the Oil Filler Outlet Tube and the Air Cleaner Inlet Tube, I chose a table napkin which I could separate into thin sheets.   I cut a strip from one thin sheet in order to have something I could hold while testing.

Following David Brown’s advice, I started the engine and tested while it was not yet at operating temperature.

I removed the Breather Hose between the Tube at the Oil Filler and the Tube at the Air Cleaner.

I held the long strip of test paper to the Tube at the Oil Filler.  The paper flapped vigorously.  Actually, I could sense the output of Crankcase air pressure with my bare finger!  It was a pretty good flow of putt-putt-putting air.

Next, I put the Test Strip to the Air Cleaner Inlet Tube.  Nothing—or almost nothing.  I then cut a small square from the test paper which would cover the Tube’s opening.  The paper would stay on the Tube—just barely.  I could not sense any vacuum at this Tube when placing my finger at (but not on) the Tube.

Holding the square of tissue just barely away from this Tube, vacuum would not pull the tissue onto the Tube.  Very little vacuum!

From this primitive test, I deduce that the Crankcase output is sufficiently strong to propel the Vapors through the Hose and into the Air Cleaner.  The vacuum at the Air Cleaner Inlet Tube is very weak but does not inhibit the intake of the Vapors.

David and I discussed the weak vacuum at the Air Cleaner Inlet Tube.  We agree that the large volume of air being drawn into the Twin Air Delivery Tubes of the Air Cleaner precludes high vacuum at the Air Cleaner Inlet Tube.

I could not sense, with this crude test, a difference at Cold Engine and at Warmed Engine.


  • David Brown continues to help us to test and discover.
  • Our daughter, Janeva Sulman, made the video possible.
  • Glory, the German Dappled Dachshund, belongs to our daughter
  • Breather Hose Volkswagen Part #:   N203741
  • The Hose (on my Beetle) is approximately 17-3/8ths inches long/44 centimeters long.

I did a top-end over-haul on this original factory H0 engine about 16 years ago.  I could find no evidence in the Breather Hose of any Crankcase oily residue, thankfully.

Posted by Jay Salser

My wife, Neva, and I have been driving and working on VWs since 1976. In fact, we raised our family in these cars. Now, we are retired and enjoy VWs as a hobby. The ’67 Beetle always has been our favorite year. We own a '67 Beetle and a '68 Karmann Ghia.

  1. Good one, JK!

  2. The daughter and I had fun putting this together, with the help of our German Supervisor, Glory, of course! Just an inexpensive way to demonstrate air-flow. The Son and I did such a test for one of my demonstrations showing that air is pulled through the Generator by
    the Fan of the Fan Shroud. Fancy testing equipment can measure exactly, but often we don’t need to know “exact” statistics–just know that things happen and then to demonstrate the purpose of that “happening”.

  3. Really enjoyed the video!

  4. Hello Jay. Glad to see you looking so healthy. Another fine ’67VW article on how to “test” our VW without all the fancy/expensive testing equipment. Take care.

    1. Hello, Felix–it is SO good to hear from you! I hope that you and Aurora are staying well. Is the Black ’67 also doing well? Thank you for reading and for your comment. I have fun with these little experiments. Each time, I learn something new! Hopefully, somewhere down the road we’ll see one another. In terh meantimne–keep enjoying your Beetle! jay

      1. Now I’m going to test my own Beetle.

        1. Have fun with this test. Wanna borrow our daughter’s doggie? She’s sure to “point” you in the right direction, Eric!

  5. Always good to have a German expert on hand. Thanks for the Video Jay!

    1. Good to hear from you, Jim! Yes..those German Experts give us such confidence! hahaha Thank you for reading, then commenting! It makes the Volkswagen World go ’round! jay

  6. Enjoyed the video and the written post. Helpful for me, thanks. I want to share a company who helps me with retaining my VW’s tip-top shape. “Northside Autohaus”. They have spectacular car maintenance services that specializes on Volkswagens and Audis. Thanks again!

    1. Hello, Emma…Thank you for your review of and comments to the Video! I believe that you are in Australia? The Link which you sent did not copy onto the Comment Site–just to a reference message which I received. Maybe you can send additional information with a reference to the actual location and address–I think that that will copy onto Comments. I am happy to hear that you have located a good shop which cares for its customers. Such shops are becoming fewer and fewer, it seems. In my particular area there is but one–unless I want to drive for many miles to another. Thank you for reading–then commenting! Keep enjoying your VW! jay

  7. Jay, Thanks for your cool stuff. Take care.

    1. Hello, Bruce–as always, it is good to hear from you. I have fun doing these little experiments–the family often joins me and we have fun. Our children literally grew u p in t he back seats of our Beetles and our Karmann Ghia. As they got older, they would hold lights, fetch tools, etc.. The wife was thus relieved of some of her helping. Came time for driver’s licenses and the children continued their VW experiences–even how to diagnose problems and to duplicate noises they heard while driving. We still talk VWs! jay

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