Understanding the MK7 PCV function, retrofitting the MK8 PCV, and why catch cans are bullshit(IMO)

[DerHase]

Enjoy. A lot of this was gone over in my build thread (see sig) as well, if you feel so inclined to check that out.

This was the video that kicked off the now 2+ year long learning process for me (and everyone else along the way).

Edit 7/13/25: Doing some tidying up because this first post was a real mess. The thread is now 77 pages (and counting). All of the most important information is summarized here because it got so ridiculous I had to make a website to structure it all... most recent updates/information will always be posted here:
https://datadrivenmqb.com/mk8pcv

Edit 8/31/25: About 2 years later I think we're finally at a decent point knowledge-wise to establish some general rules of thumb when it comes to choosing PCV upgrades. This turned into a LOT more than just MK8 retrofit-specific stuff... this is really what you need to know regardless of what you're considering doing with the car:

Here is the PCV diagnostic flow chart mentioned in the above video - I threw this together to best summarize *MY* thought process as it pertains to solving PCV problems on the MK7 platform based off the last 2 years of learning, collecting data, and connecting the dots to arrive here:

Extra credit: Here are some other important bits which led to everything learned above. These are suggested if you really want to nerd out and/or see the process of it all:

All MK7s are not created equal. Know which parts your car was built with to have some kind of idea of what will or won't work:

Turbo inlet pipes are important to the stock MK7 valve working. This also displays how the Venturi (full retrofit) can overcome the vacuum source problem when non-OE TIPs are used:

Direct on-track testing with a non-OE TIP in MK8 basic vs full retrofit form... this is really neat to see the difference in real time, highly recommend:

At one point I tested an APR PCV plate... it's good stuff and while it doesn't maintain vacuum on the crankcase at idle, it was found to not be near as bad as I had expected. Great option for track use if your specific use case and/or engine code dictate it being the next most optimal choice:

If all the above isn't enough... I have an entire PCV playlist which contains even more boring stuff if you want to see the rabbit hole I went down.
MK7/MK8 PCV Tech Playlist

 

[mrmatto]

Amazing work on all of this. A huge contribution to the community. Fascinating findings on the “paths” through the system.

One minor gripe: Are catch cans bullshit if they stop the smoking on the track, even if the reason why they’re working is different from what’s commonly understood? They probably aren’t doing what sellers claim they’re doing, but they are solving(ish) a problem. Your document explains the theory pretty thoroughly.

Regardless, a topic worth continuing conversation on and expanding our understanding!

 

[DerHase]

Amazing work on all of this. A huge contribution to the community.

One minor gripe: Are catch cans bullshit if they stop the smoking on the track, even if the reason why they’re working is different from what’s commonly understood? They probably aren’t doing what sellers claim they’re doing, but they are solving(ish) a problem. Your document explains it pretty thoroughly.

That's fair. I think what they MIGHT be doing is keeping that vacuum drawn on the TIP outlet of the PCV just long enough to "balance out" the sudden rush of air from the cyl head "blue path". The way it MIGHT help I suspect is completely coincidental.

Can and line volume will have a big effect. One might even be able to "tune" the volume to minimize oil being sucked straight into the can, while also not allowing (much, maybe even any) oil to enter the blue path.

I do think that the oil separator and bypass is the primary benefit to us, and the "steps" leading to the blue path are an additional measure.

As full disclosure, my MK7 valve is used, but shows no other signs of problems. I got the KR issues when I added another 0.6ish deg camber and moved the sway bar to full stiff. Never once had a problem on the street, etc.

I have stuff on the way to be able to datalog pressures on Simos Tools directly with the other engine parameters, so it'll be much better for assessing changes. Once that's set up, inplan to swap valves around again, and also have an APR CC on the way from a buddy for testing purposes.

 

[mrmatto]

That's fair. I think what they MIGHT be doing is keeping that vacuum drawn on the TIP outlet of the PCV just long enough to "balance out" the sudden rush of air from the cyl head "blue path". The way it MIGHT help I suspect is completely coincidental.

Can and line volume will have a big effect. One might even be able to "tune" the volume to minimize oil being sucked straight into the can, while also not allowing (much, maybe even any) oil to enter the blue path.

I do think that the oil separator and bypass is the primary benefit to us, and the "steps" leading to the blue path are an additional measure.

As full disclosure, my MK7 valve is used, but shows no other signs of problems. I got the KR issues when I added another 0.6ish deg camber and moved the sway bar to full stiff.

I have stuff on the way to be able to datalog pressures on Simos Tools directly with the other engine parameters, so it'll be much better for assessing changes. Once that's set up, inplan to swap valves around again, and also have an APR CC on the way from a buddy for testing purposes.

Yeah I think it would be great if your depth into how the PCV works and your CC theory could help someone (APR? 034? EQT?) develop something to more directly and precisely do the job.

Looking forward to more data and how it evolves your understanding.

 

[DerHase]

Yeah I think it would be great if your depth into how the PCV works and your CC theory could help someone (APR? 034? EQT?) develop something to more directly and precisely do the job.

Looking forward to more data and how it evolves your understanding.

Thanks!

Best part of all of this is that the entire retrofit is <$200. $120 of that is for the 2021 Tiguan hose, and $3 for the sealing bung are basically one-time expenses. After that the MK8 PCV assembly is $60-65 for replacements. It's cheap enough that I purchased a second one just to have on hand as a spare. Also have another hose on the way that I ordered to forward to a friend in Canada (because apparently that part is on backorder up there and he can't get it) which I'll be blowing some air through to "bench test" how strong the venturi effect actually is before either buying an 034 TMD or physically putting a fitting in my turbo discharge pipe to run boost through the (currently unused) top fitting of the hose.

I also forgot to mention that of all the inline catch cans - BMS is probably one of the best because it's a tiny can, and the hose runs are very short. It would be my first choice with a stock MK7 PCV if tied to that by auto-x rules (that went out the window with my hood vent though).

I'm hopeful that the Radium offering will be good. It is far different than every other valve currently on the market. Based on their teaser photo, this is what I can speculate about it:

Red is oil/blowby mixture being pushed up through the red path in the VW diagram, and oil returned through the AOS like stock.

Blue is the cyl head "blue path" that every single other PCV plate MFR does away with. It appears to go through the labrynth of the plate and ultimately is drawing on the AOS fed by the red path. Relatively similar concept as a stock PCV valve.

Yellow is drawn on by the TIP. Where it differs from stock is that instead of pulling from the typical AOS area (upper left in the above photo, rear of the valve on driver side as mounted in the car)... it is drawing through a second AOS, which is pulling vacuum on the cam cradle area, not specifically the lower end where the blow-by gasses originate. I don't think this is necessarily a bad thing, but it may cause a slight delay in the TIP actually drawing a vacuum on the crankcase. Unfortunately this is something I don't foresee being able to test accurate, not will I go through the trouble because it would require measuring crankcase pressure/vacuum directly in the bottom end which will not be nearly as simple as modifying the oil cap. Many engines vent/draw suction exactly like this though, so assuming their PCV plate functions otherwise, and doesn't have any weird shortfalls, I speculate it won't be an issue.

What I'm unsure of, is how they are intending to regulate vacuum from the blue path at idle/low load (assuming I'm interpreting their diagram correctly). The stock valve utilizes the large diaphragm to limit how much vacuum is actually pulled on the crankcase.

I had to noodle on the above thought for a little while - because since both the blue path port and the TIP draw vacuum through that diaphragm... and where I was measuring vacuum was at the oil cap (so vacuum draws through the valve, down the red path, crankcase, and then up to the cam cradle by the timing chain)... how was I getting different readings at idle and WOT in the crankcase?

My best guess is that the check valves physically don't allow enough air to pass when the blue path is pulling air through them... that they can't pull a full vacuum on the valve (in the case of the MK8 valve). The MK7 valve I was seeing more vacuum from the blue path than I was at a TIP, which is more "expected" since the TIP will never generate anywhere near 20in-Hg of vacuum.

 

[DerHase]

My best guess is that the check valves physically don't allow enough air to pass when the blue path is pulling air through them... that they can't pull a full vacuum on the valve (in the case of the MK8 valve). The MK7 valve I was seeing more vacuum from the blue path than I was at a TIP, which is more "expected" since the TIP will never generate anywhere near 20in-Hg of vacuum.

Coming back to this, in fact if this were not the case then the blue path would just be a full on 20in-Hg vacuum leak through the TIP outlet.


My spare MK8 PCV and Tiguan hose just showed up... Time for more bullshitting in the garage tonight ?

 

[DerHase]

?

Should be nice having multiple "configurations" to test once I do get it on track next month. Looks like it'll be a very adaptable setup depending on vehicle specs that can be messed with to fine tune.

 

[YamR1rider]

Thanks for taking the time to document and present it all. I've always been of the mind that catch cans which retain the stock PCV are pretty pointless, but can see the point of a plate replacement type for heavily tracked cars to address the cloud of smoke issues - so was particularly interested to hear your downsides for them.

 

[evilhomura89]

Thanks for sharing your findings. I didn't know this PCV retrofit was possible as it came with a sensor and all, but thanks to your hardwork we know IT IS POSSIBLE

I've always been a fan of these OEM-plus kind of retrofit, it's like a minor upgrade we can get our hands on easily. And i also believe engineers at VW try to solve earlier issues with revised parts and all, hence any new parts that fits properly is always interesting to me.

What's your view on upgrading to the mk8 PCV on a stock daily driven mk7 GTI without any oil consumption issue?

 

[DerHase]

Thanks for sharing your findings. I didn't know this PCV retrofit was possible as it came with a sensor and all, but thanks to your hardwork we know IT IS POSSIBLE

I've always been a fan of these OEM-plus kind of retrofit, it's like a minor upgrade we can get our hands on easily. And i also believe engineers at VW try to solve earlier issues with revised parts and all, hence any new parts that fits properly is always interesting to me.

What's your view on upgrading to the mk8 PCV on a stock daily driven mk7 GTI without any oil consumption issue?

Thanks, but the credit goes to @Mosquito who has been running this setup for nearly a year now. I'm just the first to test it in an autocross setting (and soon to be track setting next month) and document everything.

Honestly if you don't have any problems I wouldn't fuck with it at all. If your stock PCV isn't causing oil burning issues, I wouldn't screw with it. That said it's <$200 for everything, and future PCV replacements are (at this time) only ~$60-65ea. The hose is the most expensive part. If one wanted to retrofit this without utilizing the venturi, if someone could source the TIP connector and PCV connector separately, a much less expensive retrofit hose could potentially be made. The Tiguan hose is ~$120. And the sensor sealing cap is ~$3.



In other news:

I'm eagerly awaiting the 034 TMD to arrive so I can move on with actual on-car testing.

Main things that would hold us back from using the PCV "fully" retrofitted will be if the "boost leak" is too big. I "finalized" my tune until this PCV stuff is sorted (mainly by dialing in my WG base settings) to establish a "baseline" of what kind of WGDC is needed with the MK8 PCV valve as previously retrofitted, with the top hose removed and capped off. Also introducing a leak will likely delay spool to some degree. So I did a quick 3rd gear pull a couple days ago to collect the following data:

Graph data for dummies:
- I'm going WOT at ~2000rpms according to the dash gauge. Obviously not super duper accurate way of doing it, but doing it roughly there and monitoring when the throttle plate ACTUALLY opens fully (I'm using anything >70% as the criteria) we can get 2100rpm as the "start" of when to measure change in spool.
- I'm monitoring PUT and PUT SP... and once PUT = PUT SP (defined as within 0.5kPa) is where we're determining the end of "spooling".
-AAT was 81F for reference, IATs were 107F and dropped to 94F during the pull. These affect spool and WGDC, but as long as I can do my testing in +/-5F weather once the TMD arrives I think it'll be within noise.
- So we've got roughly a 700rpm spool time, and from 5000-6500rpm we have an average 74% WGDC as baseline measurements.

I'll be doing the same thing for whenever I have the PCV retrofitted after playing with the WG tables.

Problem with anything affecting the WG final duty cycle is that if it deviates too far you will get an EPC light. We've gotta establish if this is a 2-3% hit in WGDC... or more like a 10-15% hit. Outside 10% tends to need the tune adjusted. *IF* it's at least a "consistent" leak... because of the PID calculation that the WG base vs final works off of... it will still function just fine even if it's a consistent 10% off base vs final... the gate adjustment will not be bouncing all over the place and should still hold PUT at PUT SP.

 

[scrllock]

Thanks, but the credit goes to @Mosquito who has been running this setup for nearly a year now. I'm just the first to test it in an autocross setting (and soon to be track setting next month) and document everything.

Honestly if you don't have any problems I wouldn't fuck with it at all. If your stock PCV isn't causing oil burning issues, I wouldn't screw with it.

One thing I've not seen mentioned here is that the head is super soft and easy to strip. I think Mosquito's reluctant to pull his PCV again for fear of needing a helicoil. My Mk8 will be the fourth PCV on this head and they're for sure not meant to have those screws in and out as often as we're doing.

 

[Mosquito]

Absolutely and a good heads up.
The threads are super soft. Factory torque for the self taping screws is 9nm which don't bottom out. OEM PCVs don't leak oil at the seal, so don't over torque them like you do with the aftermarket plates.
On aftermarket plates. Air pressure cleaning the threads between switching goes a long way to keep them in good shape.

FWIW: My threads hold about 5nm top right now. I cannot remove my PCV without having to helicoil. This is after about 2 1/2 years of playing around with solutions and about 10+ installs.

 

[Mosquito]

On the MK8 PCV.
I am a believer that it is a far better PCV, even in a DD than any prior OEM option we've had and in 95% of the cases, it also far exceeds aftermarket plates as well. (Short of the unreleased Radium, the R&D of current aftermarket plates is shameful)
One thing to note is that the hose and its proper use is part of the magic. For now, we use the Tiguan hose w/ hose#2 capped or in the case of DerHase, will use the venturi in its full form soon. Can't wait to see the results.
If you want to try other things, by all means but start from stock so you can see when you are moving in the wrong direction. Trust that VAG engineers know what they are doing.

Personally, I tracked charcoal canister tank ventilation fuel vapor through the vacuum ports (was an issue of mine), IAT differences across plates (FL summer related), carbon buildup at the valves and exhaust tips (leaking valve stem seals), oil seepage through seals (crankcase pressure) and a few others things which slowly allowed to build up an understanding of how I thought this thing worked. About 8 months in sr1981 and I came across the technical doc which confirmed we were on the right track.

 

[00Zero]

I eagerly await your results, I think the PCV is going/gone on my 1.8T and this looks like a worth while upgrade when I do the repair.

 

[reverend_sean]

One thing I've not seen mentioned here is that the head is super soft and easy to strip.

Absolutely and a good heads up.
The threads are super soft.

This is a great info share. I have been swapping between PVC (winter months) and RL catch can (track months). I have not noticed the threads weakening, but I have only done the swap 2 times.

@DerHase if you wanted a RL catch can system to play with, my last track weekend is 9/16-17 and I would be happy to send it to you for science.