I have a 472-cubic-inch engine in my ’69 Cadillac and I have a problem with oil vapor covering my nice new aluminum valve covers. I had a PCV valve on the engine, but at highway speeds it would sometimes push out the dip stick so I removed it and just put a big breather on the valve cover. But even with a piece of cloth (it’s just an old sock!) around that big breather, it still drips oil on my valve covers. I was thinking of buying one of those big oil separators. What should I do?


Jeff Smith: Sometimes the smallest details can make a big difference in a daily driver. Not everybody is concerned with peak horsepower. Often, it’s just a matter of getting the car to perform the way you want. Let’s start with a quick review of what the PCV system is and how it works.

Before 1962, cars merely dumped crankcase vapors overboard. If you look at old photos of the freeways in Los Angeles from the early ‘60s, you will see a black stripe of oil centered in each lane from the oil dumped on the road from these road draft tubes. The very first emissions device was the positive crankcase ventilation (PCV) valve. Its job was to direct a measured amount of vacuum from the intake manifold into the crankcase, pulling oil vapor into the intake manifold where the oil was burned.

At one time there were probably hundreds of different PCV valves – all specific to individual engines. But today, the valve has been consolidated to perhaps a few dozen applications. The idea is great but sometimes these valves just don’t work, or if they do, they do the barest minimum job of pulling engine vapor from the crankcase.

It’s also possible for a PCV valve to work too well. If the PCV valve pulls too much oil, this can be worse than not pulling enough. Often pulling too much can be caused by how the PCV valve is positioned in the valve cover. There should always be a small housing or vapor separator chamber between the PCV valve and the open portion of the valve cover. Without a baffle, a good PCV valve will pull liquid oil directly into the engine which is obviously not good.

With a baffle in place, the routing of the PCV valve circuit is important. It starts with a filtered, fresh air inlet. Most factory systems route a line from inside the air cleaner to an opening in the valve cover. On the opposite valve cover, a PCV valve is routed with a hose over to manifold vacuum source – often found on the back side of the carburetor. So the system pulls a small amount of vacuum from the intake manifold, past the PCV valve and pulls in fresh air from the air cleaner. This creates a full circuit that will purge combustion vapors out of the crankcase.

This assumes that the PCV valve is doing its job. It might take experimenting with several PCV valves to find one that actually works well. We ran across a small company called M/E Wagner Performance out of Pennsylvania that sells an adjustable billet aluminum PCV valve. This might seem an odd thing to do but engineer Matt Wagner discovered that many PCV valves really don’t do their job.

Matt and his dad Gene designed a PCV valve that is adjustable, allowing the end user to create the ideal amount of ventilation through the engine. The valve is not inexpensive but it will certainly do the job.

This is M/E Wagner’s billet aluminum and adjustable PCV valve. It allows you to custom tailor the amount of vapor pulled from the engine.

Often, pulling a large volume of vapor out of the engine will also mean that inevitably some liquid oil will accompany the vapor. This can mean employing a vapor separator or catch can in between the PCV valve and the connection at the engine. Moroso makes a really nice aluminum air/oil separator that features a valve opening at the bottom to drain the collected oil. Inside this separate is just some steel mesh that helps separate the oil from the vapor.

We got to thinking about this, and I think if you were handy, you could make your own vapor separator. My buddy Tim Moore’s idea was to use an AC system’s large aluminum receiver/dryer canister. These are generally filled with a desiccant that removes moisture from the refrigerant. His idea was to find a suitable unit at the junkyard for $5 and cut it in half with a band saw, dump the desiccant, and then make a retainer that would be filled at the top with coarse steel wool. Then drill the unit for inlet and outlet fittings along with a drain plug at the bottom. To re-assemble the unit, you could have it TIG welded – or we thought you could glue it back together with a high-strength epoxy. I think JB Weld would do the trick since this can isn’t highly pressurized.

These are just some ideas for how to build a well-designed PCV valve that will operate the way it’s supposed to. The best part is that this system will keep the inside and outside of your engine clean at the same time.

Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.