1978 DeVille - PCV valve function

[Koa]

Hi all,
on each valve cover there is a device for PCV breathing. What is the function of both of them?
Thanx

[The Tassie Devil(le)]

G'day Karel,

On one side, there should be the PCV Valve, which is routed to the full Manifold Vacuum, and on the other side, there should be a port/fitting that connects up to the Air Filter.

What happens is that the air filter side allows filtered air into the internal s of the engine, inside the Sump, and the PCV Valve is a regulating fitting that sucks contaminated air, as in excess blow-by from the undersides of the pistons, or valve stems in order to relieve pressure being built up within the crankcase.

This contaminated/polluted air passes through the combustion operation, and out the exhaust pipe.

If you have the same things on both sides, then someone has done the wrong thing, and swapped parts without realising what they are doing.

Bruce.

[TJ Hopland]

Right valve cover has a rubber elbow which holds the PCV valve itself then a 3/8" line that runs to a port at the bottom front of the carb.  78 likely has the electric compressor if its got air shocks so no other connection in that hose.    Left valve cover has a grommet then a rectangle periscope looking box that contains a filter that goes between the valve cover and air filter housing.   

If yours isn't as described someone has done some very misguided modifications.   This isn't some horsepower robbing emissions thing.  Not having a PCV will make for very nasty engine internals. 

The valves are easy to get still because they were not that engine specific.  Elbow also not super specific.  The filter can is getting hard to find but can be cleaned if needed. 

If your stuff is missing I would see if you can find enough parts to get it back working again.  Even aftermarket carbs have the port.   For the inlet filter you can use and aftermarket one or even the more common to GM of the era meant to fit inside the air cleaner housing,  it was like a plastic tray with the element stuck in it and a nipple on the back,  you can shove that right into the valve cover.   Won't look right but will work fine. 

[Koa]

Both things are in place as You describe.
The question is, how should the PCV valve work. While having it pulled out I can see a smal piston in it with very low tension. When  I try to breeze towards carburetor, its free, when I breeze towards engine it also allows a little of air to go through. Is that correct?

[The Tassie Devil(le)]

The valve has the spring-loaded disc that flaps around, but I have never bothered to suck or blow through one.

But, as you say it moves, then it would appear to be good.

They work as I described previously.   No real need to reinvent the wheel.

Bruce.