davewh

I have a 67 327 with the pcv valve (ac delco cv726c) on the oil filler tube when you disconnect the pcv valve from the oil filler tube and put your finger over the hole on the threaded side what is the difference in engine rpm at idle with the finger on and off?
Thanks
Dave
Just trying to learn

Avispa

Couple hundred RPM. Not much air flows through the PCV valve when there is high manifold vacuum on the outlet side of the valve. Please continue to use it. PCV valves serve an important purpose in getting blow by gases out of the crankcase before they get themselves dissolved in the engine oil. Even though they started out as a pollution reduction device (to stop venting crankcase gas directly to the atmosphere), they turned out to be a valuable tool to reduce sludge buildup and acid accumulation (from the burned sulfur in gasoline mixing with water vapor in the combustion blow by to form sulfuric acid) in the oil.

MikeM

You should see a minimum of 50 rpm drop when you put your finger over the end of the valve. Any less and you have a wrong or plugged valve.

I've never seen a spec of the maximum rpm rise but I suspect from experience it's not much more than 50 rpm if you're using the correct valve.

Originally, these valves were calibrated for a particular engine's vacuum characteristics. Also some valves are installed vertically, horizontally or even on a 45*. I'd think that would have some effect on the operation of the enclosed spring/valve but don't know for sure.

If you have an OEM system, you should be able to lay a piece of paper across the dip stick tube opening and vacuum should hold the paper in place.

wmf62

Bill

davewh

Thanks for the information, I also though that you should see about 50 to 60 rpm, when I do it I get about 200 rpm change and do not know if that is normal.

MikeM

Well, I'm going to "guess" that either your carb is midadjusted on the idle screws OR, your PCV is stuck open. Does it rattle when you shake it?

Clean it out with some carb cleaner and see if it makes a difference. I think the original tuneup specs called for changing that valve at regular tuneup intervals or at least checking it.

There was probably a reason for doing this way back with the oil available then but I have a couple of valves that are almost 50 years old and still working properly. Cleaned them once that I can remember.

davewh

It is a 2 year old valve and yes it does rattle. I also did clean it with cleaner. I am not sure how much it should seal at idle I know some air should pass.

Frankie the Fink

I would have said something a bit higher than 50 RPM. I'd try it on my original '61 and tell you but not sure its a great indicator as my RPO-242 (the PCV option) is a cobbled together version of my own and I wouldn't swear to how well it's calibrated. In addition to the PCV valve I had to fabricate a restrictor to go in the hose on the crankcase side to get the car balanced out the way I wanted it. I suspect my STANDARD SV-100 PCV valve was not really designed for my solid lifter car so I
had to Bubba somewhat.

But it sure does the job for me !

MikeM

The valve has an orfice in it to let it pass air at idle. I believe the valve is closed at high vacuum idle which overcomes the spring. Low vacuum/open throttle, the spring pressure pushes the valve full open to overcome blowby.

The 50 rpm rise spec comes from the 1963 New Product Information training booklet published by Chevrolet Motor Division. Other years may be different but in my experience, most brand cars that I fooled with in that era had a 50 rpm rise under the conditions you described.

65tripleblack

The valve never closes against intake manifold vacuum. The higher the manifold vacuum, the harder the pintle valve pulls to elongate the spring and the tapered channel shows less and less cross sectional area as negative pressure increases. At manifold pressures near atmospheric, as in WOT, the spring relaxes to its free length, which fully opens the passage cross sectional area.

The only time the valve closes, is when the pressure in the intake manifold goes positive, as during a backfire. When this happens, the pintle travels in the opposite direction, and its flat back side seals positively against the internal passage. This is for prevention of a crankcase vapor explosion.

davewh

My understanding is the same as MikeM.