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

Added Excerpt from Factory Drawing showing variations of PCV, including B20A which were not imprted into the USA.  

The PCV system is occasionally the source of some confusion because you will notice that between the B18 and B20 diagrams, the direction of flow 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 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.  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.  

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

Reference Information


B18 PCV  

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

Source:  1966 122S owners manual



B20 (carbureted) PCV

Source:  120 Series Intereurope workshop manual 162


B20 (fuel injected) PCV 

Source:  1971 1800E Service Manual


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 freah 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


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.  Not shown, but this would have also been combined with a plain, vented Oil-Filler Cap.  Picture credit: Derek. 

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-Factory (Creative) PCV Plumbing:  

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. 



Reference Information:  

Oil Trap pipe dimension on B20. 


Crankcase Pressurization: 


In this excerpt of a factory diagram, 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. 

B18 OCV:


B18 PCV:


B20 (Carbs, single or dual):  


B20 (F.I.):



External material sources are attributed.  Otherwise, this article is Copyright © 2015-2018.  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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