Positive Crankcase Ventilation (PCV) valves
 

I have two different PCV valves purchased recently.
One was sold by one of the MV (mainly Jeep) parts dealers out of the US. (If they are like most old Chev parts dealers all of the repro parts are made in the same factory no matter how they are packaged so I don't think the name of the vendor matters, in any case I'm puzzled not angry). This version is marked AC1543018 Ordnance 338400 and came in a box labelled Valve, assy WO-A6895 FM-GPW6769.
The other version was an ebay purchase that looked similar so I thought I would take a chance. It came in a box marked as AC Ventilation Valve CV-476 5747235 and repeats these markings on the valve.
Photos of both PCV valves are attached to this post and the next two. Both externally resemble the later production valve used on CMPs.
The description from the manuals is "In operation the high vacuum in the intake manifold at low engine speed overcomes the tension of the valve spring causing the valve to seat in the housing restricting the orifice. As the engine speed increases, and the manifold vacuum decreases the valve spring forces the valve off its seat thereby increasing the size of the orifice" (around the valve plunger).
My puzzle is that this makes perfect sense to me for the silver CV-476 valve but on the black WO-A6895 valve there is a drilling through the middle of the plunger body that to me would mean the valve is effectively always open, not just at higher engine speeds as described.
It also looks as if I could swap the CV-476 guts into the black housing to have both cosmetics and function, but I haven't tried this to see if it fits and functions.

Photos of the silver CV-476 valve
 

This shows that when held to the light, there is no open passage.

Photos of the black "Jeep" valve
 

Including an image showing there to be a passage open at all times.

I'm not familiar with PCV valve design, but I'm guessing it's a bypass passage designed to provide a degree of crankcase ventilation at high vacuum, ie. under light throttle. In other words, a transition passage between idle and load conditions, just like carbies have for fuel metering. Obviously it would act as an air bleed at idle, but presumably it's small enough to be compensated by readjustment of the idle speed and mixture screws. Anyway it's obviously designed that way so it would be worth trying before changing the plunger.

Ford CMP PCV valve setup
 

Not familiar with the Chev Blitz PCV valve setup but here are two types that I know of used on Ford sidevalve/flathead V8 engines.

One type has the PCV valve feeding directly into the manifold at the rear side of the carby and the other has a spectacle piece between the carby and manifold and the PCV valve is fed into the front side of this piece just below the carby.

This front feed is the same as shown in my Ford CMP parts and maintenance manuals.

Hope this is of some interest.

Cheers,

Jacques

Grant.

Just wondering if you had any luck with regards to these PCV valves? I’m missing one for my engine

Jordan, This might help? The Ford flathead ones rely on gravity (have no spring in them) so they have to be mounted vertically (and the right way up)

Jordan,

The attached shows the Chevrolet setup. This valve also has no spring inside it, but is helpfully marked with an arrow next to the word 'UP'. It is a sealed unit, but I believe there is a later type that screws together. I could find no part number on my valve, but it is marked with 'Donaldson Co. Inc., St Paul, USA', if that helps.

Check out sections K4 and K5 of the MB-C1 maintenance manual for some more info, including a schematic of the valve.

Owen.

markings but no part number
 

Jordan

I have one (and am also looking for a replacement) similar to what Owen posted the picture of.

Markings on it are:

near the "UP" - " DONALDSON CO INC ST PAUL U.S.A. "

near the bottom - " U S PAT. 2120050 CAN PAT 378550 "



Brian