Why do we even need a PCV system?

Why do we even need a PCV system?

Everything you need to know about the benefits and purpose of a fully-functional 1.4L Turbo PCV system and why you shouldn't just bypass the whole thing or vent to atmosphere.

Why do we even need a PCV system?


This post assumes that you know what a PCV system is and the basics behind crankcase ventilation. In addition, the post is focused around the GM 1.4L Turbo's PCV system, but applies to effectively all turbocharged engines. Feel free to use this post in any PCV-related discussion, as the principles are fairly universal.
To explain this, I created a logical diagram of a modern turbocharged engine PCV system, which will be referenced in this post. C refers to Condition; so C1 is Condition 1.

Part 1: Primary Vacuum Source

The first benefit we'll discuss is the vacuum produced in a normal PCV system. When the engine is under deceleration, idle, or cruising, which most turbo engines operate under when driven in normal conditions, the intake manifold applies vacuum to the crankcase. Following the diagram from the Intake Manifold backward, we have vacuum pulled through a check valve, through the PCV chamber (which can include a series of plumbing connections), through a regulator diaphragm, through an AOS, and into the crankcase. Ultimately, the intake manifold applies vacuum to the crankcase. This is important for a number of reasons.
  1. Reduced oil consumption. When the crankcase is under vacuum, that vacuum assists the piston rings with keeping oil from seeping into the combustion chamber, which reduces oil consumption. Furthermore, it keeps oil on the crankcase side of the turbo seals and valve stem seals. This is particularly effective with low tension piston rings that rely on the assistance of crankcase vacuum to provide an optimal seal. The first thing noticed when the PCV system begins to malfunction is an increase in oil consumption. In the Cruze, oil consumption on a low-volatility engine oil like AMSOIL's Signature Series is under 1/2 a quart per 10,000 miles when the PCV system is working correctly.
  2. Increased power and efficiency. On the same note as above, low tension piston rings rely on crankcase vacuum to provide an optimal seal. In fact, this is so significant that in some racing applications, the switch to low tension piston rings and significant crankcase vacuum can provide substantial power gains. Make no mistake, OEMS are aware of this and have applied these principles in modern engines to chase every last bit of efficiency possible to meet CAFE standards. If crankcase vacuum improves piston ring seal, it therefore also improves compression. 
  3. Reduced external leaks. On the same note as above, seals are less likely to seep oil if there is vacuum behind them keeping the oil on the inside of the engine as opposed to the outside.
  4. Release of low vapor pressure liquids (fuel and water). Anyone that has attempted to boil water at altitude will remember that water boils more quickly and more easily at altitude, where pressure is lower, and the same concept applies to engines. Smaller turbocharged engines are particularly prone to fuel dilution due to the rich air-fuel ratios required under power enrichment modes to keep the combustion chamber and catalytic converter temperatures under control. Fuel starts boiling at 95F and fully boils at 395F, and water (introduced through condensation during heat cycles) boils at 212F at atmospheric pressure. Applying vacuum to the crankcase effectively reduces those boiling points and accelerates the vaporization of those liquids. Once vaporized, those liquids are evacuated through the PCV system and consumed by the engine through the intake manifold. The reason all of this matters is because fuel dilution reduces the viscosity of the oil and compromises its film strength, and water creates acidity and depletes the detergent package of the oil. Being able to evacuate these contaminants significantly increases oil life and therefore service intervals.

Part 2: Vacuum Control

Having established the benefits of applying vacuum to the crankcase, we should then ask, "how much is enough?" Older piston-style PCV valves operated on the basis that a crude piston inside a cylinder was loaded against a spring, and the vacuum of the intake manifold applied against the piston would compress the spring. These "PCV valves" served not only as valves but also regulators, but unfortunately applied very little actual vacuum to the crankcase; approximately 1-3 in-wc vacuum (inches of water). While this is better than nothing, modern turbocharged engines require notably more crankcase vacuum. In the case of the Cruze and many others featuring a vacuum regulator diaphragm, a large, spring-loaded diaphragm replaces the piston normally found in a PCV valve, and regulates crankcase vacuum independently of the one-way check valve. In the case of the GM 1.4L Turbo, that vacuum is regulated to 11-18 in-wc at idle. Note that excessive crankcase vacuum can cause seal failure, which on the 1.4L Turbo starts at the front crank/main seal.

Part 3: Secondary Vacuum Source

This is a less well-understood topic, but is worth considering when designing PCV systems for aftermarket applications or when diagnosing issues on modern PCV systems. When the intake manifold is under vacuum (C1), we have vacuum on the crankcase and all is well, but what happens when the intake manifold is under boost? In the case of the Cruze, the check valve at the intake manifold closes, and PCV gas is routed through a large hose to the turbo inlet. This is an important routing, because the turbo inlet is also a vacuum source. This location is right between the intake on the turbo and the air filter, where the restriction of the air filter, produced by the turbo (effectively an air pump) creates a vacuum. While this is not as strong of a vacuum as what is created by the intake manifold under C1, it is nonetheless a vacuum source that provides some of the benefits mentioned above. Side note: on newer engines, OEMs are even adding air pumps to provide more consistent crankcase vacuum while the intake is under boost.

Part 4: Wrapping It Up

The biggest question I get asked on a regular basis is why one couldn't just vent the whole system to atmosphere with a breather. Assuming the rest of the PCV system is left alone, venting to atmosphere effectively creates a vacuum leak. When using breathers with check valves, the vacuum leak is negated, but a potential failure point is added to the system in the event that the check valve leaks. Furthermore, a well-designed PCV system should never produce a high enough crankcase pressure that a breather is required in the first place. I've found that the vast majority of the time, these breathers are used to band-aid broken or poorly designed PCV systems. If the rest of the PCV system is bypassed with a breather, you lose all of the benefits of applying vacuum to the crankcase while also exposing the crankcase to more condensation and noxious fumes in the engine bay to be picked up through the cowl.
This article was written to answer some of the common questions that come up regarding PCV systems. Hopefully, what was explained above helped you understand the role of each component and the benefit it offers to the overall system. While the implementation of each component by the OEM may have its flaws, the functionality it provides is technically sound. A working PCV system minimizes oil consumption, reduces emissions, improves efficiency, improves performance, reduces external leaks, and extends oil life.
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