Check Valve on PCV Inlet

Aug 25, 2018
South Carolina
I'm going to do an experiment on my 2002 Tahoe with the 5.3L and 276k miles. I have a check valve that has a cracking pressure of ~3 psi (~6 inHg) and I have an open orifice (no plunger) PCV valve. I'm going to put the check valve in the inlet hose. The idea is to control the air flow on the inlet side instead of the exit side. The intake vacuum should pull a slight vacuum on the crankcase up to the check valve cracking pressure. Fresh air will still be allowed in after the valve opens.

I have no idea what I hope to accomplish by doing this. lol I'm just bored, but I think this will be fun to see how it affects oil pressure and probably test how well the seals are actually sealing. I doubt 6 inHg will be enough to cause any sealing issues. Maybe it'll reduce windage and aeration slightly? Maybe a slight fuel economy boost in the fractions of 1 mpg that I could never accurately test within a margin of error? I don't know. I figured I'd share this with you all and get some input.
Years ago I did this on a 1979 Accord.
It had bad valve guides and seals, so it smoked when idling.
It had only one breather vent from valve cover to a port downstream of the air cleaner.
Not a dual passage like modern systems.
I rerouted that through a pcv valve and segment of skinny hose to further limit flow, to manifold vacuum.
That pulled vacuum in the crankcase and stopped the smoking.
Got it through inspection, which did not include emissions check.
Didn't leave it on for fear of running the valve guides dry.
It will be interesting to see what, if anything, happens. Please keep us updated. The crankcase vaccum pumps used in racing came to mind when I read your post.

The article lists improved ring seal and less windage as benefits, with a target figure of 12-14" of vacuum. The difference, though, is that the race application pulls vacuum and vents the blow-by to the atmosphere. PCV systems are meant to pull blow-by into the engine and burn it again. One problem with a check valve could be that in low vacuum situations, the engine might make more blow-by than the PCV can handle. With the inlet sealed, it can't go anywhere.

I figured out blow-by pretty quickly since my first few vehicles were '70s cars that became mine well past the salad days. I bought a Slant-six Duster that ran like crap on the way home. The PCV inlet went from the valve cover to the air cleaner, and the filter element was soaked with oil and caked with dirt.

After a few years, I was fighting an off-idle stumble. The Holley 1-barrel had a little spring-loaded plunger on top of the float bowl and a 3/8" hose running from the top of the carb to the fuel pump housing. When the throttle opened, a lever closed the plunger. Without the Internet or a repair manual to troubleshoot with, I finally figured out that crankcase pressure was blowing into the carburetor when that plunger was open, and that caused my stumble starting off with a manual trans. Removing the hose and plugging both ports made it run right again. The system was originally supposed to pull gas fumes from the carb, through the block, and into the PCV.

On my hotrod Pontiac, of course, I had to have an open air cleaner element. That meant getting onto the throttle filled the cabin with blow-by that would have gone down the carb with a stock air cleaner.
I run crankcase vacuum in my drag car so I'm familiar with it.

This Tahoe spends 90% of its life cruising down the highway at 1800-2500 rpm. Since pumping loss is the biggest factor in fuel efficiency, I'm curious if drawing a slight vacuum on the crankcase would help with that. If the intake has 15 inHg vacuum at cruise, but you're drawing 6 inHg vacuum on the crankcase, the cylinders are seeing 9 inHg vacuum instead of 15 inHg.

What effect would it have on oil oxidation and service life?
It's installed. I checked the cracking pressure on the valve before putting it on and it was right on 3 psi. After installing it, my warm idle oil pressure dropped proportionally from 42 psi to 39 psi so that tells me it's working. Of course the gauge is referencing ambient air pressure. The oil pressure relative to crankcase pressure is still the same.