Positive Crankcase Ventilation: More Important Than You Think

by | Nov 2015 | 0 comments

It’s been around for over five decades, but do you ever take it into consideration? Ignore it at your peril.


A PCV valve is a very simple mechanical device, but if the system it’s part of isn’t right it can cause problems that range all the way up to serious engine damage.

PCV (Positive Crankcase Ventilation, not “Pollution Control Valve,” as some oil company ads of the 1960s would have had us believe) is one of the most ignored engine systems.  When it’s working properly, you won’t even notice that it’s present, but if it’s not it can cause problems that range from poor idle quality and serious oil leaks to sludging and internal engine damage from oil contamination. Every piston engine no matter how well designed and assembled has a certain amount of blow-by past the rings, and if it can’t get out of the crankcase pressure will build up and water vapor, excess gasoline, and combustion contaminants will collect in and dilute the motor oil.


This diagram illustrates the flow of outside air through the crankcase to pick up blow-by and carry it to the PCV valve and into the engine to be burned during the combustion process.

Before automotive emissions control became a big issue, a road draft tube system was used to vent the crankcase gases straight into the atmosphere. This tube was a simple pipe that extended from the upper block to an area below the bottom of the engine, and its outlet was typically cut at an angle. When the vehicle was driven at highway speeds, the air passing by the end of the tube created enough vacuum to draw the blow-by out of the crankcase. Fresh air entered through a mesh-filled oil filler cap usually found on the valve cover.  This kept pressure from building, but at slow speeds and idle no vacuum was present, so air flow was minimal and moisture and harmful acidic vapors tended to build up inside the engine.


These illustrations show the different flow positions of a PCV valve.

In the 1950s, concerns over the heavy smog that hung like a shroud over Los Angeles and wasn’t doing the health of the populace any good prompted research into the automobile’s contribution to its formation.  It was determined that up to 30% of vehicular air pollution was the unburned hydrocarbons and other gases and vapors that were emitted through the road draft tube. You could actually see (and smell) fumes escaping at both the road draft tube and the oil cap, especially if the engine was in bad shape. This visible air pollution was unacceptable in a society that was awakening to the damage our industrialized way of life was doing to our planet. So, PCV was mandated in California in 1961, and nationally in 1963.  It wasn’t a new idea even then. Long before emissions control became an issue, some military and commercial vehicles used it to cut internal engine deposits.  It’s been an unmitigated success, eliminating a great deal of a vehicle’s overall air pollution while keeping the inside of the engine much cleaner than it would be without it (it was a major factor in the adoption of the long oil-change intervals we have today).

Location and Fresh Air

PCV Cutaway.

We cut this one open so you can see how the spring and pintle interact. This is the maximum flow position.

The PCV valve is commonly plugged into a grommet in a rocker or cam cover (an internal oil separator or baffle keeps liquid lubricant away from the inlet of the valve) and is connected to engine vacuum through a port into the intake, typically under the carburetor or throttle body.  This vacuum draws a metered flow of blow-by, oil fumes, moisture, and fresh air from the crankcase into the intake stream so that they can be incinerated and evaporated in the combustion chambers. The source of the clean air that replaces the volume of vapors removed evolved somewhat since the inception of the original PCV system.  Early on, it came through a mesh in the oil filler cap as it did with the road draft tube set-up. On a V8, this was usually found on the opposite-side valve cover from that in which the valve was installed.  In 1968, the “closed” PCV was introduced.  In this simple modification, the clean air was picked up through a plastic mesh filter inside the air cleaner housing.  That way, if blow-by should ever exceed the capacity of the valve it would back up through the vent and still be taken into the intake stream.

Varying the Flow Rate


The PCV valve spring on the left is stiffer than the one on the right. The spring rate controls how much vacuum is needed to move the pintle to the low-flow position.

Engine rpm, vacuum, and load all affect the amount of blow-by gases produced, so PCV valves are designed to vary their flow accordingly.  Typically, there are two flow-rate modes for a valve. The first mode only allows a small amount of gases and vapors to pass at idle and low-load cruising when vacuum is highest and blow-by is lowest, while the second mode flows a larger amount when the engine is under load during acceleration and high speeds and vacuum is relatively low. The original equipment PCV valve works quite well with the specific engine package it was designed for, but due to part number consolidation many aftermarket replacement valves don’t match the flow rates of the stock unit.


Here are the insides of the M/E Wagner adjustable PCV valve.

If any performance modifications have been made that change the vacuum characteristics of the engine, or even if the stall speed of the torque converter is causing low vacuum at idle in gear, the valve flow rate may not match the needs of the engine. We have been using PCV valves that were designed for engines with a “hot” cam, such as a Boss 302 Ford or a Chevrolet Z-28, in many tuning jobs. The brand of valve we’ve been using is AC Delco because the letter on the bottom of the valve’s pintle indicates the flow rate, while many others don’t. The letter “F” on a CV-769c indicates a medium-flow valve, and the letter “H” means it’s got low flow. The PCV valve should be in the low-flow position at idle including when the engine is in gear if it is equipped with an automatic transmission.

If the PCV valve is pulsing from the low-flow to the high-flow position at idle, it’ll cause rough, uneven running.  If it’s not functioning correctly and the flow rate is too high for the needs of the engine, it will in effect be an uncalibrated vacuum leak that can cause lean running at idle. The disadvantage of using a low-flow valve is that it may not have enough capacity for an engine that has a lot of blow-by.  If you’re working on a high-performance engine with a hot cam that needs a high flow rate, the answer may be to try an adjustable PCV valve.

The adjustable valve that we use is available from M/E Wagner Performance (https://mewagner.com) of Bear Creek, PA. It is adjustable for the idle mode (high vacuum), plus you can tune the cruise portion (medium vacuum) to come in when the valve transitions from idle to cruise based on the vacuum characteristics of the engine you’re working on. This is a great idea. The valve comes with instructions for easy adjustment.


These pintles range from low-flow on the left to high-flow on the right.

Vac Source

The vacuum source for the PCV valve should be a place somewhere in the intake tract that will allow the incoming crankcase vapors to be equally distributed to all the cylinders. The most common vacuum source is a 3/8 in. tube located on the base plate of the carburetor or throttle body.

[alert variation=”alert-info”]NOTE: Some newer aftermarket carburetors have a 3/8 in. tube under the primary fuel bowl that is intended for PCV vacuum, but that is necked down to 1/4 in. In our opinion, this may not allow enough volume to keep a high-flow PCV valve operating properly. If you’re connecting the valve to the intake manifold itself, it should be in the plenum. If it’s connected to a port on a runner that goes to one or two cylinders, it may cause those cylinders to not have the same air/fuel mixture as the others, resulting in unevenness. [/alert]

This is a low-flow valve in the open position.


This is what a high-flow valve looks like.

Valve Covers and Oil Consumption


This necked-down nipple on the base of a carburetor may not be the best vacuum source for a PCV valve because it may not allow enough flow for proper operation under load.

The PCV valve should always be located in an area that allows it to draw the vapors from the crankcase without exposing its tip to liquid oil or oil spray. The most common location is in the valve cover of a V8 or V6 OHV engine, but there’s a whole lot of engine oil floating around in there that’s being thrown off from the rocker arms. Ford, General Motors, and Chrysler all use baffles or oil separators in their valve covers so that the PCV system won’t pick up the liquid lubricant along with the blow-by.  Some aftermarket valve covers have good baffles/separators, while others are marginal in this regard.  If the engine you’re working on has high oil consumption, or the spark plugs are getting oil fouled, the problem could be that the tip of the valve is being exposed to too much motor oil.

As already mentioned, the engine needs a vent system that allows clean air into the crankcase. The original-equipment hose connected to the clean side of the air cleaner usually works fine, but a K&N type crankcase vent filter can also do the job.
PCV systems continue to evolve.  Some late-model engines, such as those from Volkswagen, get pretty much all the oil mist out of the stream of crankcase gases with convoluted passages and even cyclonic separators.  That kind of sophistication isn’t necessary with traditional OHV V8s.

oil separator baffle

This separator/baffle does a very good job of keeping the oil that flings off the rocker arms from entering the PCV valve.

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