Positive Crankcase Ventilation (PCV) Diagrams and Notes
First published 2015 R. Kwas  updates on-going  [Comments Added]

The PCV system is occasionally the source of some confusion because you will notice that between the B18 and B20 diagrams below, the direction of flow (of gasses) at oil filler cap changes (and the PCV Valve is eliminated on the B20).  The direction is not critical, in either case, the important thing/principle for a properly working PCV system is a filtered fresh air source to mix with crankcase gases, and a (flow-limited) path to the intake manifold to extract the mixed gases for burning

Note that plumbing of PCV Sys is often combined with that of a Brake Booster, which also needs Intake Manifold Vacuum to operate...there is nothing wrong with this as long as direction of flow, flow limitation, and safe conditions if a backfire were to occur in the Intake Manifold are observed and satisfied.

Problems occur typically when the extraction path is blocked (fouled PCV valve [No.4 on B18] or flame guard or nipple [B20]), this leads to crankcase pressurization and oil leakage around Oil-Filler Cap or other points of easy exit.  

Note:  Before there was Positive Crankcase Ventilation (PCV), there was Open Crankase Ventilation (OCV). 

B18 PCV Configuration
B20 (Carbureted) PCV Configuration
B20 (Fuel Injected) PCV Configuration

PCV Rattle Trap

Additional Information
    Open Crankcase Ventilation (OCV)
    Oil Filler Caps

Non-Standard Crankcase Ventilation
Example of Creative (but wrong!) PCV Plumbing

Reference Information
    Oil Trap Pipe Dimensions
    Crankcase Pressurization

    PCV Options  [These are excerpts from factory drawing showing variations of PCV, including B20A, which were not imported into the USA.]


B18 PCV Configuration:    

1.  Flow limiting orifice.
2.  Metal mesh filled Oil-Filler Cap
3.  Oil Trap to Intake Manifold connection
4.  PCV (one-way) valve  (see also: 
PCV Rattle Trap)
5.  Flame guard
6.  Oil Trap

Source:  1966 122S owners manual



B20 (Carbureted) PCV Configuration: 

Source:  120 Series Intereurope workshop manual 162


B20 (Fuel Injected) PCV Configuration: 

Source:  1971 1800E Service Manual

See also PCV Options below.


Good info copied from an excellent site on the D-Jetronic Injection:  http://members.rennlist.com/pbanders/PCV.htm  

PCV Valve Operation Modes

High Manifold Vacuum, Low Crankcase Pressure

This mode corresponds to the idle condition. The pressure differential presses the disk against the intake manifold side seat, where the metering slots permit a regulated flow of gasses into the intake manifold.

High Manifold Vacuum, Moderate Crankcase Pressure

This mode corresponds to over-run (coasting in gear with the throttle closed). In over-run, manifold vacuum can exceed 20 in. Hg. The pressure differential presses the disk against the intake manifold side seat, where the metering slots permit a regulated flow of gasses into the intake manifold. Blow-by is minimal as combustion is at a low level.

Low Manifold Vacuum, Moderate Crankcase Pressure

This mode corresponds to part-load conditions. The pressure differential presses the disk against the intake manifold side seat, where the metering slots permit a regulated flow of gasses into the intake manifold.

Very Low Manifold Vacuum, High Crankcase Pressure

This mode corresponds to heavy load to full-load conditions. Here, most of the pressure differential that opens the PCV valve comes from crankcase pressure. The pressure differential presses the disk against the intake manifold side seat, where the metering slots permit a regulated flow of gasses into the intake manifold. If the blow-by volume exceeds the ability of the PCV  valve to draw in the vapors, the excess blow-by flows back through the crankcase fresh air intake system to the air cleaner box, where it is pulled through the throttle body and into the cylinders.

High Manifold Pressure

This mode corresponds to an intake backfire condition. Here, the high positive pressure in the manifold presses the disk tightly against the crankcase side seat, sealing the PCV valve and preventing flame propagation into the crankcase to prevent an explosion.



Link to interesting Brickboard thread:  http://www.brickboard.com/RWD/index.htm?id=920391

Link to related thread for replacing the fresh air hook-up from front filter with a mini fresh air filter mounted on the oil filler cap :  http://www.brickboard.com/RWD/index.htm?id=1184285  ..this prevents condensate and/or oil in the front filter!

Not an uncommon posting, showing what happens when PCV is clogged, resulting in crankcase pressurization and oil leakage:   http://www.brickboard.com/RWD/volvo/1439938/140-160/oil_leaking_like_crazy_front_timing_cover_felt_seal.html


PCV Rattle Trap https://forums.swedespeed.com/showthread.php?593015-PCV-rattle-trap

My response:  "PVC Valve rattle is a condition (annoying, but otherwise not really harmful) which occurs when the frequency of pulses of vacuum at idle is equal to the resonant frequency of the return Spring and Mass of shuttle/ball in the Valve, in combination with the volume inside the PVC plumbing...its a pretty specific confluence of conditions (like hitting the mechanical lottery!) that happens occasionally (but only at idle, where frequency of the individual pulses are slow enough, and don't yet combine into a steady vacuum)...change any one of the critical parameters, and it will stop...so much for the theory (my good friend was a physics teacher, so I had to get this right or I would never live it down!)...

in practice, the simplest solution to prevent resonance, is to just change the exciting frequency a bit, by just changing the idle RPMs a little (that's what did it on my 122/B18)...if that winds up being too high or too low to be an acceptable solution, another possible preventative is limiting the excitation pulses the valve is seeing, can do it too (by adding a partial flow blockage [washer] with a hole in-line into the PVC plumbing to partially orifice block it...to see if this might help, simply squeeze the line partially shut to try it)...or creating a little buffering volume in the PVC plumbing by installing two flow limiting orifices, one at the manifold, one at the Valve, or plumbing in another length of (dead-ended) tubing with a "T" (kinda like the expansion tank on a furnace), finally, just replace the PVC Valve...what are the chances that replacement Valve will be resonant also...but where's the fun in that? Edit: I guess you already tried replacing it, and it still rattles...try replacing with a different style/manufacturer Valve, or I guess it would be time to try one of the other suggested solutions...


Additional Information

Open Crankcase Ventilation (OCV)

Before approximately 1964 (market dependent), the crankcase was vented to atmosphere by way of a tube, with no oil trap, but with tube shaped with an uphill section to prevent oil from simply running away to be lost, followed by a downhill section, which finally opened to atmosphere next to the oil-pan.  This tube was paired with the plain, vented Oil Filler Cap.  Both are visible in this picture by Geir V. 



Another variation of OCV, this time again with an open down-tube, but with an Oil-Trap at the crankcase.  This would have also been combined with a plain, vented Oil-Filler Cap, as shown.  Picture credit: A. Majerus. 



Oil Filler Caps were part of the Crankcase Ventilation System (Open or Positive).

Two types of Oil Filler Caps were fitted.  The earlier plain top one, shown on the left, filled with a filtering metal mesh, and which allowed flow in both directions by way of under the flutes, and the later, also filled with mesh, which had a fitting for the PCV plumbing.  Direction of flow in the later Cap was determined by the configuration of the rest of PCV system, as shown above.     

Oil Filler Caps.  Picture credit:  Derek


Shown here is an Oil Filler Cap with a separate filter installed.  This is a favorite solution for allowing the crankcase to breathe fresh, filtered air, without allowing oil to get on induction Air Filters (which can happen during High Crankcase Pressures - see above, or high blow-by), or when a fresh air fitting is not available, such as on non-original equipment induction Air Filters. 

Oil Filler Cap with mini fresh-air filter.  Picture credit:  Derek


Non-Standard Crankcase Ventilation: (Sometimes they get it right, sometimes not!)

Creative and Acceptable PCV Plumbing: 

A nicely racing-prepared '69 Amazon, driven in the Norwegian historic racing series.  The car was shown at the annual Oslo Motorshow.  We can see a custom vented alu PCV catch container in the normal Battery location.  Note also the Dual Weber side-draught carbs with velocity stacks, necessitating a custom inner fender clearance pocket.  Cable activated Clutch and dual Brake Master with a tandem Booster are also in evidence. 

Erik Skifjeld Endre picture used with his kind permission.

  Custom PCV System for vintage competition.


Creative (but wrong!) Crankcase Venting Plumbing, can cause symptoms affecting idle, to oily gunk in carbs or Air Filters, to oil leaking from Dipstick, to name just a few!  Here are some examples of "owner engineered versions" of NSCV (Non-Standard Crankcase Ventilation). 

On the non-standard arrangement shown below, upper (at Oil Filler Cap), and lower (at Oil Trap) are tied together.  Without further modifications, this would allow Crankcase pressurization and leakage, but the careful observer will see the Oil Trap has also been modified with an additional downvent (at Yellow) which allows venting of the Crankcase...either to atmosphere, or into a catchbottle required by competition rules, so this non-standard version would actually be acceptable! 



Example of Creative (but wrong!) PCV Plumbing (from the Let's-just-tie-all-of-the-hoses-together-and-forget-about-it-school-of-PCV-Plumbing):  

Shown here is an example of PCV plumbing which is wrong, because it allows Crankcase pressurization, which results in oil weepage...and results in oily gunk in Air-Filter. 

Here, all hoses are tied together, and there is no PCV Valve or flow restriction in sight.  From thread:  https://forums.swedespeed.com/showthread.php?608263-minor-leakage-around-oil-dipstick-66-P1800&p=7555119#post7555119   Picture by "Breaker" and used with his kind permission. 

"...oil leakage coming out from around the dipstick." ...complaint was likely a result of Crankcase pressurization
under certain engine operating conditions of this creative and non-standard arrangement. 



Reference Information:  

Oil Trap Pipe Dimensions on B18 and B20. 


Crankcase Pressurization: 



Thread:  minor leakage around oil dipstick - 66 P1800: 



PCV Options:

In the following excerpts of a factory diagrams, many of the components are shown for the various configurations, including some, like the B20A (single carb) which were not available in the USA.  Shown are the common configurations. 

In all of the following diagrams, Blue highlights Filtered Fresh Air and Orange indicates Crankcase Gases

B18 OCV:


B18 PCV:


B20 (Carbs, single or dual):  


B20 (F.I.):



External material sources are attributed.  Otherwise, this article is Copyright © 2015-20232.  Ronald Kwas.   The term Volvo is used for reference only.  I have no affiliation with this company other than to try to keep its' products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you donít, youíre just a lazy, scum sucking plagiarist, and I hope your B18 ingests a brick!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment. 


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