The Importance of Oil Quality with PCV Systems

The Importance of Oil Quality with PCV Systems

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Did you know that your oil quality affects your PCV system? Oxidation stability matters, and here's why.

The PCV system is generally used to evacuate crancase pressure, which should be a given since it's called "positive crankcase ventilation." However, in the process of evacuating blowby gas, it also evacuates other crankcase contaminants, such as oil vapor and water vapor.

The Process:

When oil is heated over extended periods of time, it vaporizes. This is referred to as the oil's volatility. Oil vapor makes its way through the PCV system, where it coats the PCV components along the way and in the case of flat areas, can settle and accumulate. This accumulation causes the oil to "sit" for extended periods, where it slowly thickens due to oxidation and eventaully creates hard, crusty deposits. Furthermore, condensation (introduced as a byproduct of combustion and as moisture during heat cycles) is also evacuated through the PCV system, where it can yet again accumulate in flat areas. When the oil that has settled in the PCV system is exposed to water, oxidation is further accelerated.
Depending on the quality of the oil you're using, you can end up with PCV tract that looks something like this:

On the 1.4L Turbo engine in question, PCV byproducts tend to settle in two locations; the PCV tract of the intake manifold, and the PCV path going through the cylinder head between the valve cover and the cylinder head. The picture above was sent by an automotive shop that called me for assistance diagnosing a PCV-related issue. Their customer came in with a vehicle that had high crankcase pressure and blowby, yet had a new valve cover and PCV pipe. I explained the path of the PCV system and diagnosed the issue over the phone as an obstruction/blockage in the PCV section. I got that picture 15 minutes later. PCV deposits had blocked off the entire PCV path in the cylinder head and intake manifold. Once cleaned, the engine operated perfectly again.

Why Oil Quality Matters

Because you can't easily control where PCV byproducts settle in the PCV system, you can at least make an effort to control the quality of the oil and the rate at which those bypdroducts create deposits. There are two particularly important metrics in engine oil that will affect the rate at which these deposits build in the PCV system, volatility and oxidation.
  • Volatility. This metric is included in practically all engine oil specifications. It is technically referred to as NOACK volatility, standardized under ASTM D5800. Volatility refers to the rate of vaporization of engine oil. Under ASTM D5800, A standard volume of oil is heated to a standardized temperature, then held at that temperature for a set duration, after which the oil volume is measured, and any loss is reported as a percentage. API SN (practically the most barebones specification out there) limits volatility at no higher than 15%, while GM Dexos1 reduces that further to 13%. Most of the Group III based "synthetic" oils on the market are somewhere around 10-12% NOACK volatility. Volatility is a direct contributor to PCV deposits (and oil consumption) as it directly contributes to the loss of oil from the crankcase, which then travels through the PCV System. Reducing volatility has a direct correlation to reduced PCV deposits. However, a reduction in volatility is typically accompanied with a higher quality oil, where an improvement in oxidation stability also exists.
  • Oxidation stability. This metric is also included in engine oil specifications in various forms. Oxidation refers to the chemical reaction between the hydrocarbons in oil and oxygen. The oxidation stability of the oil is a function of the base oil's quality and the strength of the antioxidant additive package. Oxidation is accelerated by heat and exposure to oxygen and water, as well as acidity and other catlysts. The direct result of oxidation is a thickening of the oil and eventually the formation of sludge or deposits. This is also referred to as "coking." While volatilty refers to the volume of oil vapor that will be passed through the PCV system as a consequence of the oil's vaporization, oxidation refers to what will happen to that oil once it gets into the PCV system and settles somewhere. The better the quality of the base oil, the slower the rate of oxidation, and therefore the slower the rate of deposit formation in the PCV system. has an excellent article on this subject that explains this behavior in more detail:

Our Recommendation

In my experience, having spent 6 years studying automotive tribology, I have found that higher quality oils to have inherently better volatility and oxidation. Off the bat, I recommend against any oil that isn't at least advertised as synthetic. I recommend avoiding any conventional or "semi-synthetic" oils for any turbo engines due to the inferior volatility and oxidation stability of those oils. While a decent "synthetic" oil will give you a volatility of 10-12%, which is notably better than the minimum required by GM Dexos1, you can still do better. I personally use recommend, and sell AMSOIL Signature Series, which as of today has a volatility of 6.7% in a 5W-30 and an oxidation stability that is among the best in the world. AMSOIL's use of high end base oils (which I believe are a blend of PAOs and Esters) and antioxidants offer exceptional volatility and oxidation stability even at long (25,000 mile) service intervals. My customers have noticed far cleaner PCV systems and a significant reduction in oil consumption over the course of an oil change interval. Check out AMSOIL's Preferred Customer program for big discounts and free shipping.

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