Q&A / Tech

Ask Away! with Jeff Smith: How to Choose (or Build Your Own) PCV System to Control Oil Vapor

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.

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  1. The M/E Wagner unit has worked beautifully on my .680 lift solid roller 439cid big block Chevy street-strip engine in my 1970 Chevelle. No more oil vapor smell at stop lights or oil dripping from valve cover breathers. Engine has only 4″ vacuum at idle so no other pcv valve from the oem manufactures would work. GREAT people to deal with! Never too busy to answer your questions.

  2. AnalogDan Wilson says:

    There are way too many automotive performance enthusiasts that think positive crankcase ventilation (PCV) systems are strictly for reducing the amount of potentially harmful vapors that are allowed to escape an engines crankcase into the atmosphere, contributing to the air pollution problem.

    That’s exactly what a PCV system does but some hot rodders are under the misconception that the system will rob their engines of horsepower. That’s simply not true and some engines could even see an increase in power caused by better piston ring sealing due to negative pressure or a vacuum in the crankcase.

    Automobile manufacturers have installed PCV systems on millions of cars for the obvious reason of reducing hydrocarbon vapors released into the air that we breathe. Even new engines with the best sealing piston rings available will still allow a small percentage of unburned fuel vapor to get past the pistons and accumulate in the crankcase.

    The harmful stuff in the crankcase gets worse during normal operation. High cylinder pressures encountered during combustion also cause blow-by that’s loaded with abrasive carbon particles that contaminate engine oil over time. Moisture in the air will condense with each heating/cooling cycle of the engine and form small amounts of water inside the crankcase. Add in the oil vapors that are released from oil at high temperatures and then the conditions are right for damaging sludge to form inside your expensive engine.

    This potential problem is easily eliminated by using a simple PCV system that’s designed for the vacuum characteristics of the engine it’s used on. My experience is based on the 351 Cleveland 4-V engine in my 1970 Mach 1. The common PCV valve part number for a STOCK 351C 4-V engine is the EV-50. But the extensive modifications including a radical solid lifter roller cam and 4.56:1 differential required experimenting with different valves and the weights inside the valve that determine the actual opening and closing events of the valve based on the overall intake manifold vacuum.

    The somewhat expensive but technically impressive adjustable PCV valve offered by M/E Wagner that the well respected author, Jeff Smith mentioned in his story, will look great on my engine because its technical features will suit my requirements perfectly.

    I said all of that other stuff just to say this….ALL ENGINES Need a proper PCV system for environmental reasons, optimal performance and longer life. I welcome any replies that offer a legitimate reason NOT to have one. And the beat goes on….

  3. Nick Wheaton says:

    Put a vacuum pump on it.

  4. Jim cronin says:

    I am trying to find a data sheet on older PCV valve. Has anyone come upon a listing of this sort?

  5. stihlydan says:

    I don’t seem to see any reference to an engine needing crankcase vacuum to function properly and thus needs to have a PCV to do so. The need to not pollute is the last thing that was thought of with a functioning PCV system. Look at outdoor power equipment for example. They never had pollution in mind when they started doing theirs. Like all internal combustion engines, they need to have crankcase vacuum. Without a functioning PCV system, oil can be pushed out just about any mating surface. If not for a valve that allows positive crankcase pressure to escape and seals for the negative pressure, this vacuum would not happen. I’m pretty sure the original intention was for vacuum and the fact that it recirculates the oil vapor was a secondary benefit.

  6. Jeff Smith,
    In the OP, a question was asked about oil coming out of the dipstick at highway speeds with an operating PCV system.
    What is your opinion about this? Thanks.

    • Jeff Smith says:

      It would appear that there is sufficient pressure in the crankcase to push the dip stick out of the tube. I’ve seen this on small-block Chevys with very weak rings and insufficient crankcase ventilation. If the engine has a PCV valve – likely it does not work or is too small to adequately ventilate the crankcase.

  7. Mark Heath says:

    I have a 5.0 ford after I installed a supercharger the oil would come out of the dipstick and reroute the valve cover breather to the intake however is this going to damage my engine

    • most likely have the hose under the s/c , means you are not getting a vac signal ,..boost also !! you will need the pcv source to be before the s/c .

  8. I have a Subaru EJ25 non-turbo-2001
    I am so confused with all the non-scientific posts on other forums and many opinions are not based on any measured degree.
    My question for the boxer motor is – under normal driving conditions would the engine be damaged by venting to atmosphere and blocking off the manifold PCV with breather filters on the top and either side of engine to the valve cover breather positions.
    Thanks in advance.

  9. Jim Clements says:

    If you route crank case pressure through steel wool, don’t vent to the intake manifold!!! Particles of steel wool will corrode and break off. You don’t want those little buggers making their way into the cylinders.

    • Joseph Hansen says:

      It’s not really steel wool, like you get at the hardware store. It’s closer to a steel mesh. I think it’s too heavy to allow small particles to corrode and break off.

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