Learn me: crankcase ventilation systems without PCV valve

I have this Mercedes 190E and I don't really understand how the crankcase ventilation system works. It only uses one port on the valve cover and it doesn't use a PCV valve. It is also tied-in with the idle air system.

The system looks like this:

There is an oil separator inside the valve cover. Once separated from oil, the vapors flow through the valve cover port. After which there is a T. One side goes to the air cleaner (above the CIS fuel metring plate) and the other side goes through a plastic rectrictor which looks like this:

The large hole is connected to the vacuum port and the the small hole lets the vapors through the restrictor. After this is another T. One side goes to a port on the intake manifold which splits into all 4 runners. The other side of the T goes to the idle valve. The other end of the idle valve is connected to a port located after the CIS metering plate but before the throttle body.

How is this supposed to work? At idle, the intake manifold has high vacuum and the idle valve is open. Metered air is drawn from above the throttle into the manifold. But at the same time, some blow-by gases are pulled through the restrictor into the manifold. Wouldn't that put "too much" air into the manifold to maintain the desired AFR at idle? This leads me to think that the ECU compensates for this and tells the valve to open just a bit less. Is this right or am I totally wrong?

Then at full load, the idle valve would be closed and a small part of the blow-by gases would be pushed through the restrictor into the manifold (unmetered). The most part would go back to the air cleaner where it would be metered and meet the other part of the gases that went through the restrictor. 

Isn't there supposed to be some clean air that goes from the cleaner to the crankcase? How does this system get away with all this unmetered air? What is the use for the restrictor?

Reason I'm asking is that I am replacing CIS by megasquirt. I removed the CIS metering unit and have installed a port in the new air cleaner to run the idle valve. I thought about running just a filter on the valve cover instead of pcv but that would be "cheating" as it would stink and make unnecessary pollution. Could I just connect both the idle valve and the valve cover port to a T to the air cleaner like this?

I used to think that PCV valves were just fancy check valves, but they are actually way more complicated than that.  When they are working properly they are only pulling vacuum at mid-throttle.  They block air flow at full vacuum to prevent idle issues.  At full throttle there's no vacuum anyway, thus no flow.

I don't know much about your system above, but I do know that a lot of people who use Megasquirt with aftermarket turbo systems just delete the PCV system entirely.  In addition to idle issues, most PCV valves will leak boost when the IM is pressurized.  The SOP for this situation is just to run the PCV hoses to a catch can and vent to atmosphere.  The oil gets dirty faster this way, and needs to be changed more often.

Remove the P from pcv, and you have your answer.  There is a vacuum line involved to keep things flowing, but the early Euro things are basically a breather.

ShinnyGroove said:

I don't know much about your system above, but I do know that a lot of people who use Megasquirt with aftermarket turbo systems just delete the PCV system entirely.  In addition to idle issues, most PCV valves will leak boost when the IM is pressurized.  The SOP for this situation is just to run the PCV hoses to a catch can and vent to atmosphere.  The oil gets dirty faster this way, and needs to be changed more often.

I did that before on a turbo conversion car. I just put a breather and vented straight to atmosphere. It would leave oil mist in the engine bay and smell like oil vapors when idling. I'm trying to avoid doing the same this time around!

Streetwiseguy said:

Remove the P from pcv, and you have your answer.  There is a vacuum line involved to keep things flowing, but the early Euro things are basically a breather.

Fast forward a few decades.and Chevy's Vortec 8100 truck engine's crankcase ventilation system is a 3mm hole in the valley on the intake manifold.  No hoses, no check valves.

^ I can't disagree, but a proper catch can filled with copper kitchen scrubbers can eliminate quite a bit of the oil mist and vapors.

ShinnyGroove said:

I used to think that PCV valves were just fancy check valves, but they are actually way more complicated than that.  When they are working properly they are only pulling vacuum at mid-throttle.  They block air flow at full vacuum to prevent idle issues.  At full throttle there's no vacuum anyway, thus no flow.

I don't know much about your system above, but I do know that a lot of people who use Megasquirt with aftermarket turbo systems just delete the PCV system entirely.  In addition to idle issues, most PCV valves will leak boost when the IM is pressurized.  The SOP for this situation is just to run the PCV hoses to a catch can and vent to atmosphere.  The oil gets dirty faster this way, and needs to be changed more often.

We did this on the XR4TI, just put an elbow in and ran a catch can with a breather, fixed a host of leak behavior issues.

For years vehicle ran a simple ventilation system.

 That's why there was a dark streak in the middle of the driving lane.

In reply to Raze :

I built a PCV system for a turbo application using a tee and opposing Mitsubishi PCV valves that works flawlessly.

I'll post a pic if I can find it.

Found it.

The hose going back goes into the intake. I installed a check valve there just in case but have since deleted it, and tidied up the system considerably.

The valve on the left going into the corrugated hose goes into the intake after the MAF just before the turbo.

When cruising, the one pointing back opens pulling vac from the valve cover to the intake and closing the left one. On boost the intake one closes and the crankcase pressure opens the left one venting to the intake/turbo. I did it ti replace the faulty PCV setup on a Chevy Sonic 1.4T. There's a kit available but it wasn't particularly well thought out, the intake has to come off and it costs about 5 times what this cost to build.

Pic no worky

i thought I was being intelligent and T'd my crankcase breather and my valve cover breather together and ran them to a vented catch can on my sr20det drift car, eliminating the somewhat-stock system of pcv valve  and valve cover to vented catch can. 
 

i never would have noticed how much power I was down as it ran smooth and still sounded good.  But there was a long sweeper that I just could not keep the tires spinning on whereas the week before, on t old setup, I had no problem. 
 

I had done some other work in addition to the new setup but after checking everything else to no avail, I returned it to its former config and whalah! Back to normal  

moral of the story is there is a lot going on and you may not know if you got it wrong. 

I can see the pic.

Here is the updated/tidier version

As you know we're working on basically the same fundamental project (MS'ing a 190E), so I've been trying to decide on the same thing.  For now I put a bung on my intake tube (post air filter, pre throttle body and ICV) and am running the vent straight into that.  It certainly runs fine, but I can't speak to longevity, etc.  

Ultimately I'll probably run a catch can.  I think. 

FWIW, PCV valves are not interchangable.  They are calibrated to have certain characteristics and you can even berk up an all stock vehicle by putting in a parts store valve.

That said, I'd like to say NAPA has a PCV valve for a 2000 Silverado that has a very tiny orifice on the end, like 1/8". This is a good valve to use for an aftermarket application where a normal PCV valve causes too many idle related complications.

Knurled. said:

FWIW, PCV valves are not interchangable.  They are calibrated to have certain characteristics and you can even berk up an all stock vehicle by putting in a parts store valve.

 

That said, I'd like to say NAPA has a PCV valve for a 2000 Silverado that has a very tiny orifice on the end, like 1/8". This is a good valve to use for an aftermarket application where a normal PCV valve causes too many idle related complications.

I read...don't mess with stock...mess with stock

My smileys and thumbs up did not show up...  :-)

In reply to Raze :

In a stock app, leave it OE.

When you've bunged a 2 barrel throttle body on a 2.5l Iron Duke and it won't start cold because the throttle plates have to be so shut that the fuel from the injector can't get past them, you gotta get inventive 

Don't do it this way. 

iceracer said:

For years vehicle ran a simple ventilation system.

 That's why there was a dark streak in the middle of the driving lane.

 I always look for dark spots on the concrete cause that tells me there is a bump in the road coming up, the dark spot is where the oil leaks car get dumped from the upward acceleration of the bump causing accumulated oil drops to fall.     Then there is the wisdom of motorcycle riders who know not to ride down the middle of the road on rainy days cause that's where  its continuously oily.

Knurled. said:

FWIW, PCV valves are not interchangable.  They are calibrated to have certain characteristics and you can even berk up an all stock vehicle by putting in a parts store valve.

 

That said, I'd like to say NAPA has a PCV valve for a 2000 Silverado that has a very tiny orifice on the end, like 1/8". This is a good valve to use for an aftermarket application where a normal PCV valve causes too many idle related complications.

I wouldn't say they're not interchangeable as much as there are differences, but I think if you have idle related complications you have other problems.

The Mitsu/Hyundai PCV valve with the 1/8" NPT thread on one end, and a barb on the other is handy, and available for a wide range of (displacement) applications, but you're right, if the OE system is working, don't berkeley with it.

In reply to Suprf1y :

In theory it shouldn't matter, but a lot of vehicles are sensitive to it.

Heck, GM took to riveting the valves in from the inside of the valve cover, specifically to keep people from swapping out the valve with one of unknown calibration.  I strongly suspect this also is why Ford hid the PCV on the Duratec (four and six) to where you need to remove the intake manifold to get to it.  Too many parts stores who want to sell a PCV valve with every air filter, causing more problems than they "solve".