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

[fretburnr]

Seems to be running fine in various conditions, so at least I didn't forget to connect anything.

@DerHase Is there a PID I can log to watch PCV pressure, or was that all done with your custom instrumentation?

Otherwise, I'll just watch for any smokescreens at autocross this weekend.

 

[Lob-Star]

Trying to catch up on this thread and consume all the info. Can I avoid all of this retrofit by just venting the crank case pressure to atmosphere through an oil cap breather?

 

[scrllock]

Trying to catch up on this thread and consume all the info. Can I avoid all of this retrofit by just venting the crank case pressure to atmosphere through an oil cap breather?

No

 

[DerHase]

Has anyone ran and tested Radium Engineering Catch can ? I also see they have a bunch of V2 quick connects available.

IMO for the retrofit to the MK7 platform, using the Jack Spania Racing catch can (which is a BMS knockoff as far as I can tell) is probably the smarter route. I also personally believe it's likely the "better" option (from a routing perspective, I don't doubt that the Radium can itself is probably better at trapping stuff) - it keeps the Venturi as close to the PCV valve as possible to minimize any delay in vacuum being generated.

I'm less sure about blowing into the can post-Venturi (like the JSR can) vs sucking through the can like the Radium setup... I think if the can is really badly designed and has a ton of pressure drop across it, that sucking through the can might be the lesser of evils but that's just a hunch.

Seems to be running fine in various conditions, so at least I didn't forget to connect anything.

@DerHase Is there a PID I can log to watch PCV pressure, or was that all done with your custom instrumentation?

Otherwise, I'll just watch for any smokescreens at autocross this weekend.

You'd need to at minimum purchase the MK8 PCV sensor, and a harness from @StorableComa to go from the SAI pressure sensor to the PCV sensor. And then you need a logging solution that you can enter the custom addresses into. Simos Tools is the best way to do that. If you don't have SAI tuned out you will have a CEL but it won't affect drivability.

This is a Mode 22 list (so will work regardless of S50 or A05 software structure, and regardless of tune). But these things need to be set properly since there are kind of two steps with logging the raw SAI output voltage, and then another row to convert that to PCV pressure. It could be shoved into a single row but this helps from a diagnostic standpoint when setting the sensor up.

edit: had to correct the raw voltage formula

Name

Unit

Equation

Format

Address

Length

Signed

ProgMin

ProgMax

WarnMin

WarnMax

Smoothing

Enabled

Tabs

Assign To

Ambient Press

kpa

x / 120.60176665439

%01.1f

0x13ca

2​

FALSE​

50​

120​

70​

120​

0.9​

TRUE​

MISC

amb_press

SAI raw voltage

psi

x / 1000

%01.2f

0x40e0

2​

FALSE​

0​

5​

-1000​

1000​

0.7​

TRUE​

PCV

saivolts

PCV Pressure

psi

(saivolts * 4.7717) - (amb_press * 0.145) - 0.15

%01.2f

0xffff

2​

FALSE​

-5​

0​

-1000​

0.01​

0​

TRUE​

PCV

The red numbers will need to be changed. Get the car up to operating temp, and then shut off and check PCV pressure with key on engine off. The goal is to basically zero it +/- 0.01 or 0.02 psi or so. Play with that factor to do so. Once this is done it SHOULD be pretty close at all times because the formula also references the ambient pressure sensor to account for any kind of elevation or weather changes.

Trying to catch up on this thread and consume all the info. Can I avoid all of this retrofit by just venting the crank case pressure to atmosphere through an oil cap breather?

No. I have an opinion about people who do that which I'll keep to myself.

But in a nut shell, running a hose to the turbo inlet pipe functions absolutely NO differently than a vent to atmosphere EXCEPT when your turbo is spooled, it pulls a vacuum on the crankcase which as long as it isn't super excessive, is good.

I think a lot of peoples' PCV issues (when they have aftermarket plates) is they are running an insanely free-flowing intake setup (like the DBV2 TIP or the Blaze intake/TIP combo so as a result it doesn't pull any vacuum (or a very very minimal vacuum) on the crankcase.

As covered in the video, running the OEM TIP with the APR PCV actually pulled a ton of oil into the turbo inlet pipe, but I had no such problems with the BMS TIP which happens to have pretty close to identical vacuum levels in the crankcase at WOT as an OEM PCV valve.

There is absolutely zero benefit to venting to atmosphere instead of running a hose to the TIP. It only has the potential to make things worse.

 

[Klrider44]

Gonna give it a shot, just installed with the cut out. I was going to install the JSR can but it’s really tight back there with the blaze intake setup so I left it off for now. I probably could have made it work but things would be touching so I decided against it.

 

[DerHase]

Gonna give it a shot, just installed with the cut out. I was going to install the JSR can but it’s really tight back there with the blaze intake setup so I left it off for now. I probably could have made it work but things would be touching so I decided against it.

Yeah I was really unhappy with the way the hose is routed so I joined it to a cut up MK7 PCV hose. Obviously different setup but might make it useable for you as well?

You can kind of see what I did here.

 

[fretburnr]

Good news - no smoking at all during or after 9 runs yesterday! This is after doing the full retrofit with 034 TMD.

I didn't have time before the event to set up any pressure logging, but I noticed no issues at all even in 90F heat. I did my usual lap logging so I'll take a look at those. This course was slightly less aggressive in the braking-right-hander smoke generation layout, but I expect peak Gs to be similar to previous smoke events.

 

[DerHase]

Big thanks to @yakev724 for getting the PCV sensor added to his car and then tracking it.

This is a MK8 full retrofit (unmodified), with some kind of hybrid turbo. He had the same results (smoking) with the BMS can, though this log is without it for reference.

Very interesting stuff even without being able to see the direct crankcase pressure:

This is a big overall snapshot of his session. Notice there are a few VERY big dips into negative PCV pressure (-7 psi), followed by some positive pressure logged right after (+1.2 psi):

Zoomed in on one, you can see it's doing the exact same thing as what my car did in T17 at VIR the last time out, though to a MUCH larger extent:

If you want to see what this phenomenon looks like while driving, go to the 11:00 min mark where you can see my car do this to a lesser degree:

But otherwise looking at his log, the PCV vacuum at WOT is a little inconsistent, there are times when WOT produces only -2psi of vacuum, etc though boost is only in the ~18psi range at that point. I suspect this might be because it's a big turbo and not being fully spooled it'll be a bit more variable as to the vacuum it produces.

 

[DerHase]

Also found another local guy with a 2015 GTI who did the full retrofit, and we modified the valve prior to installing it (his was the one done in the video IIRC).

Still smokes at autocross.

Went over to his house last night and we swapped all my crankcase sensor and PCV sensor stuff onto his car so we could get actual data.

First off, here's is a pull in MY car for reference, I can log both crankcase and PCV pressures at the same time so they're both here:

He's seeing a pretty consistent -0.75psi in the crankcase, and notice that everything is way smoothed out, no sharp spikes up or down relatively speaking.

But what's interesting is the PCV sensor. On the initial hit it only gets down to -2.1psi, and then slowly drags it's way down to -3.0psi. My car with the exact same TIP and a tiny bit more boost at the same RPM was seeing -5.0psi.

His car also idles at -0.75psi in the crankcase... whereas mine sees -0.9 to -1.0. Ignore the spikes in this log to positive pressure. This was letting the car sit and idle, with my experimental diaphragm cap swapped to his car so that I could depress the diaphragm and fully seal off the crankcase and obtain a rate of pressure build-up. I need to do this same test on my own car so that it's 100% apples to apples with ventilated PCV valves.

I suspect doing this test might be a potential way to determine if the problem is a crankcase leak or if it's excessive blow by? If it takes comparatively longer to build up, then it's likely a crankcase leak. If it builds up at a really ridiculous fast rate, it might mean the crankcase is sealed but blow by may be a problem.

 

[DerHase]

... BUT he messaged me a video today and found the rear main seal was leaking. He also had some signs of leaks around the timing cover as well.

So I think now we have a pretty good idea of what "bad" in the data looks like. The PCV sensor and crankcase sensor SHOULD be able to have quick/sharp oscillations with varying demand/load. If everything is super smoothed out, then that might indicate a leak.

 

[scrllock]

what was on @yakev724 's car?

 

[DerHase]

what was on @yakev724 's car?

Full retrofit and with a hybrid turbo. @yakev724 will have to fill in on specifics though.

 

[yakev724]

dbv2 v1 (similar to vortex std) tuned for 31-33psi depending on weather. full supporting mods - MPI, RS3 LPFP, IC, downpipe, charge pipes, aftermarket TIP, stock airbox though. compression 170-172, just tested at this same track day. Latest revision oil cap.

I ran a plate cc for 3 years with no issue aside from filling it halfway each session and having to drain it/add half qt.

I've got some oil weeping around the timing cover but sounds like that's expected given I've run the plate cc for so long (not necessarily a given that it's continuing to weep, but maybe it's not a bad idea to steam clean it and observe for fresh oil now that I'm toying w the mk8 pcv).

First time switching to mk8 pcv got instant smoke on track (partial retrofit), this session was the first with the full retrofit (though car got hot + was down on power intermittently). I ran a follow up session with the BMS can inline but got same result, filled it up within a few laps.

I've just ordered another mk8 pcv to slice into and do some more controlled testing - not easy in nyc! we have traffic 24/7. Plan on going to track again in 3 weeks so motivated to find a working solution by then

 

[Klrider44]

So I installed the valve with the cutout section over the weekend and did a little bit of driving. No smoke to speak of yet, but I haven’t really put it through its paces yet. One issue I am having that is probably completed unrelated but I figured I would throw it out there just in case. I seem to be getting misfires in 1st and 2nd gear (DSG). It feels like fuel cuts to be honest. Only seems to happen at WOT in 1st and 2nd gear. Car still seems to pull very hard outside of this issue, also idles fine as well.

Could changes to a PCV valve be causing such issues? Pic of one of the captured mis fire events that shows misses across the board. I will be getting a real log at some point this week.

 

[DerHase]

So I installed the valve with the cutout section over the weekend and did a little bit of driving. No smoke to speak of yet, but I haven’t really put it through its paces yet. One issue I am having that is probably completed unrelated but I figured I would throw it out there just in case. I seem to be getting misfires in 1st and 2nd gear (DSG). It feels like fuel cuts to be honest. Only seems to happen at WOT in 1st and 2nd gear. Car still seems to pull very hard outside of this issue, also idles fine as well.

Could changes to a PCV valve be causing such issues? Pic of one of the captured mis fire events that shows misses across the board. I will be getting a real log at some point this week.

If it's ingesting oil you'd likely also see a bunch of KR as well. Feel free to send me a log of a 1-2-3rd gear pull (or individual pulls).