B6 S4 PCV issue

B6/7 S4/RS4
#1

I’m trying to figure out a strange oil consumption issue after changing the pcv.

Car has 160000 miles and has always burned a moderate, yet consistent amount of oil, about 1qt per 2500 miles. No visible smoke or leaks.

I recently replaced the pcv with an OE Audi part, the old one was obviously shot. Diaphragm was cracked and stuck.

Now, the car belches a large cloud of smoke accelerating after engine braking. Obviously, high vacuum is sucking oil into combustion chamber or manifold. Oil consumption went through the roof whilst driving with normal compression braking. When I stopped compression braking while driving, oil consumption returned to previous amount (1qt per 2.5k).

My first thought was that I installed the pcv backwards. Most pcv systems im familiar with close when high vacuum is present on the manifold side, as to prevent sucking oil into the intake. Looking at some pictures online, it appears I installed the valve correctly.

I then removed the pcv to check it’s operation, and here is where I got a bit confused. It appears to function in the opposite flow direction as I would have thought. When I suck on the crankcase side, and cover the intake side port, it closes and seals. Yet, it never seals when vacuum is applied to the manifold side… the pcv is soaking in oil, so I’m sure that is where the oil is coming from, rather than valve guides/stem seals.

What is going on here? I find it hard to blame leaking seals as it only started when I changed the pcv. Compression test shows I do have leaking rings and low compression on 1 cylinder (120psi). The rest are all 150-160.

#2

With the updated PCV unit hooked up correctly it should be like a check valve and only function when neeed.

I think a comp test is good but you might want to look into a leakdown test as that can and will overwhelm your crank case. There is a good chance this is a big part of your issue.

You can always try a VTA - vent to atmosphere approach to see if that helps. Just make sure you use a check valve for that.

#3

Thanks for the response!

The new pcv valve does fully seal as a check valve, but in the opposite direction as I would expect. It seals when vacuum is applied to the valve cover/crankcase side! It operates just as in this video. Note he is sucking on the crankcase side port. https://youtu.be/3_jeYi1myig

You mention updated pcv valve, was there a part change at some point?

A leak down test is probably a good idea. But I still don’t really understand why this issue only came up once I put in the new pcv valve.

As for a vta catch can, no check valve should be needed as it is not connected to the manifold or any vacuum source. Typically they just vent everything (blowby) from the valve cover.

#4

Ok, leak down test performed, engine is surprisingly healthy. Cylinder 1 (which was low on a static compression test) showed about a 2.5% leakdown, slightly more air was escaping through the crankcase than other cylinders. All other cylinders showed less then 2.5% leakdown . All cylinders leaked only through the crankcase.

I still do not understand the seemingly reverse to what I expect pcv valve functionality. Anybody have any thoughts?