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      11-21-2015, 02:53 AM   #1
vespa
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Exclamation DIY: Understanding the N55 PCV Valve of Doom

The N55 PCV system is permanently integrated/sealed into the valve cover and is absolutely non-serviceable but an understanding of the function and fault diagnosis may be useful to some so I will attempt to explain the method of operation. Before you shake your fist at "the man" for "keeping you down" I'll point out that the PCV valve is not only one of the most impactful and significant pollution control devices ever implemented, it also keeps fuel from destroying your oil and is one of the main reasons oil change intervals have gone from 2500 miles in the 1960's to 15,000 miles today.

If your crankshaft front or rear main seal is howling, whistling, squealing, chirping screeching or chattering like this, this, or this, check the PCV valve first. SIB 11 03 11 explains that faulty front and rear crankshaft seals were installed on all models produced between 7/1/2009 and 8/31/2011 but I suspect that many of these seal problems are actually caused/cured by the PCV valve. Photos below detail the failure of the rubber diaphragm in my 05/2011 which led to problems with both the front and rear main seals.

I don't believe there is any design fault with the PCV valve nor any reason to panic over the very high replacement cost or seemingly short life of some. I refer to the valve with "doom" for several reasons:
  1. It is easily damaged by improper boost leak testing.
  2. It can lead to some shockingly high repair bills. (Front/rear main seals can cost $1000/$2000)
  3. It can easily be overlooked as the culprit. (My dealer missed it twice)

Method of operation:

Vapors are drawn from the crankcase thru the top rear left corner of the valve cover and a convoluted molded oil separation passageway is visible leading forward to the prominent round PCV valve at the top-center, inappropriately labeled with an electrocution warning sticker. Separated oil drains back into the head thru a very small passage just prior to the PCV valve.

On the top outside of the PCV valve is a hose barb with a non-sealing decorative cover (squeeze inner sides to remove). A tiny bit of air will flow in/out of this port into a sealed chamber on the back side of the PCV valve whenever the valve is changing positions but there should never be any continuous flow. The crankcase pressure is intended to be approximately 38 mbar (0.55 psi) below the local atmospheric pressure at all times and this is maintained by a simple passive system consisting of a spring trying to open the valve while outside air pressure (thru the barb) tries to close it. When the valve is open, vacuum is drawn from the intake, and when it is closed pressure builds up from blowby gasses. The PCV is connected to the intake in 2 places: before and after the turbo -- so there is always a source of vacuum available. A simple one-way valve on each of these routes ensures that air can only flow out of the crankcase, never in. The net result is that crankcase pressure generally remains between 0 mbar and -38 mbar under all conditions. Don't take these numbers too seriously though, it's just a simple passive valve.

One of the passages goes direct into the cylinder head with no hose. The head has 6 small passages in the aluminum casting connecting the intake ports directly to a convoluted molded chamber visible along the entire left side of the valve cover. A bulge in this chamber near the PCV valve contains a simple free-moving one-way valve to prevent turbo boost pressure from entering the system.

The other passage uses the accordion metal hose at the front right of the engine. When the system is under boost there is no vacuum in the manifold so this alternate path is provided to utilize suction from the turbo inlet: Under the square protrusion at the top-front of the valve cover is a second identical one-way valve leading to an flexible metal hose connecting to the intake pipe just ahead of the turbo. Thanks to Bernoulli this junction can become quite cold so a heater is typically included on the hose to prevent ice droplets from hitting the turbo.

Along the entire right side of the valve cover there is a large vacuum tank that is completely unrelated to anything -- this was simply a convenient place to put a tank. The small shiny rigid molded hose running across the top center of the engine connects the chain-driven vacuum pump to this tank, and from here the wastegate and exhaust flap are actuated.

Failure mode:

The one-way valves are quite simple and robust, as is the sealing portion of the PCV valve itself. Clogging, leaking or gumming up of any of these mechanisms appears highly unlikely. But unfortunately the diaphragm actuator has very thin edges that can crack and leak. When this happens the valve will fail to close and the crankcase will be exposed to excessive intake vacuum, likely resulting in front and/or rear main seal squealing and/or damage.

If you have any reason to suspect PCV valve malfunction, first open the oil cap to see if the problem (e.g. main seal noise) immediately subsides -- this is perfectly safe and will confirm that crankcase pressure is related to the problem. The engine will stumble a little as it goes lean from the unexpected air flowing thru the PCV system.

You can easily test the PCV valve for failure by holding a smoke source (match, cigarette) close to the hose barb with the engine idling. If smoke is continuously ingested into the valve the diaphragm is leaking. You could also use a clear hose and see if motor oil is drawn in. Or you could apply 38mbar positive pressure to the barb with the engine off and confirm that the pressure is held for a reasonable time. Do not connect a pressure gauge to this while the engine is running, you will interfere with the PCV operation and will not get any useful information anyway.

Cause of my valve failure:

The radial direction of the crack in my valve is quite curious and does not seem consistent with fatigue so I suspect it may have been exposed to high pressure and "popped". The system is well designed such that the delicate diaphragm portion of the valve can never be exposed to raw intake vacuum or boost pressure -- only regulated crankcase pressure. And the crankcase pressure can never be too high or too low, thanks to the PCV regulator and related one-way valves. It's pretty foolproof, unless you're a fool.

I only owned the car for 4K miles when this happened so I don't know the history but I think the previous owner must have done one of two things:
  1. Connected a pump to the barb and sucked/blew hard enough to pop the valve diaphragm. I can't imagine any reason someone would do this, but I can guarantee it would break the valve.
  2. Performed a pressure test of the entire intake tract without removing either the metal PCV hose or the oil cap. Pressure tests should be performed from the turbo inlet pipe with a kit like this, but if you have an aftermarket intake with some round connection that seems more convenient, you will end up pressurizing the crankcase if you don't remove the oil cap. This occurs because you are pressurizing both PCV vacuum sources (pre/post turbo) so the air that inevitably leaks past the piston rings and PCV one-way valves will slowly pressurize the entire crankcase until the regulator diaphragm pops. It's only designed to handle 0.5psi so hitting it with the standard 20psi leak test will surely burst it.
Repair procedure:

I detailed the E92 N55 valve cover replacement procedure here:
http://www.e90post.com/forums/showthread.php?p=18946466
Cost is $400.

Some trivia about the "4 cylinder N55", a.k.a N20/N26:

Where the N55 regulates crankcase pressure from both sources, the N20/N26 only regulates when drawing vacuum from the intake manifold -- the pre-turbo pipe is completely unregulated. This seems quite reasonable since the pre-turbo intake seldom has much vacuum, and even when it does, it's only very briefly and at a time when blowby gases are at their peak anyway.

The N55 does not have true Positive Crankcase Ventilation, it literally just sucks. The N20/N26 on the other hand has an extra hose drawing fresh filtered air into the crankcase at all times. The extra flow of ventilation is likely the reason that the N20/N26 has such a large rubber PCV hose on the front of the engine compared to the small metal accordion hose of the N55.

Lastly, One of the key "advances" separating the N26 from the N20 is the new N55-style non-serviceable PCV valve. The literature explains that this is necessary to reduce evaporative emissions and is one of several tweaks that led to the SULEV rating.
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      11-21-2015, 03:46 PM   #2
Rob@RBTurbo
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Outstanding information and tutorial! Great information. It is a shame that the unit is not serviceable, however, on the N55.
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      12-16-2015, 07:23 PM   #3
Jsoboti
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If the SIB exists for it, should the dealer replace free of charge even if the car is out of warranty?
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