The great thing about the Internet is the volume of information readily available. Along with that volume often there are conflicting opinions about any particular subject. Not the least of which is the subject of ‘oil catch cans’ or just ‘catch cans’. Do you need one? I’m not going to answer that, instead I’m going to talk about what’s going on with the PCV system the catch cans are being installed in.
Pictured at left is a typical PCV valve. The first cars I worked on were late 1969 and early 1970′s cars all of which had a PCV valve nearly identical to the one at left. The PCV stands for Positive Crankcase Ventilation and valve means its a valve.
A History Of The PCV valve.
In the late 40′s a guy named Smit began studying the effects of pollution in Southern California. In his studies of smog Smit surmised the culprit was emissions from automobiles leading to smog and ozone problems. Sort of contemporaneously, but not really, in 1959 a fella from GM decided about 1/2 of the hydrocarbons identified as being emitted from cars was coming from unburned hydrocarbons. GM introduced the concept of the PCV valve in its caddy lineup and then offered the concept to everyone else. As a result the PCV valve began replacing what was here-to-for the ‘draft tube’ system, which is a whole other subject.
So what is it supposed to do?
The name itself implies what the PCV(entilation) valve is supposed to do. It serves as a valve to allow for the ventilation of crankcase vapors. A typical PCV system consists of two essential parts, an intake and an exhaust (not to be confused with the intake manifold and the exhaust manifold). In the PCV system the intake and exhaust are usually opposed to each other, meaning on a V8 the air intake is on one side and the exhaust on the other.
Typically, the air intake side comes from air that has been processed by a filter. Back in the old days the filter was often a little foamy thing inside the air cleaner usually a couple inches square inside a plastic holder clipped to the side of the air cleaner. At the other end, the PCV exhaust, a hose is attached to the induction, or intake side, of the engine. Typically the intake manifold.
Blow by: Cause and Effect
Its hard to talk about PCV systems without talking about “blow by”, especially since “blow by” is the essential reason the PCV system was put on cars. So what is blow by? Blow by is vapor from unburned gas and other contaminates the exist in the crankcase as a result of pressure bypass around the piston rings.
Essentially what this means is a high pressure explosion occurs in the cylinders and not all of the pressure is contained in the cylinder and not all of the contaminates are expunged with the exhaust stroke. Excessive blow by used to be considered a means to see if an engine was “worn out” since worn rings will cause a lot of blow by.
Okay, so now we have both pressurized, to some degree, the crankcase and introduced some contaminates. Now we need to get rid of it. Again, in the olden days we used a “draft tube” and deposited the contaminates on the ground, usually in the center of the roadway (and thus motorcycle riders avoid the center of the road, especially just after a rain).
Since draft tubes disappeared with the Edsel we now get rid of the contaminates by means of a crankcase ventilation system (note the use of Crankcase and Ventilation). This is accomplished by plumbing the pCV system into the intake side of the fuel system, ie. the intake manifold.
The purpose of the ventilation is two-fold. Well, actually three-fold. First, the ventilation system prevents the crankcase from building up pressure from the pressures leaked from the combustion process. Without ventilation you’d have to bungy-cord the oil dipstick to the motor. Secondly, the ventilation system removes the contaminates from the crankcase in order to prevent sludge from forming inside the motor.
Sludge? An interesting word. If you google “engine sludge” you will get a host of articles all related to engine sludge and contaminated oil and… and… failure to perform maintenance on the PCV system.
Next we derive at the third purpose of the PCV system and that is the emissions part. Part one and two involve pressure and the need to relieve it and preventing sludge by getting contaminates out of the crankcase. So where are we going to send those contaminates? Well, if the PCV system is functioning correctly, into the intake system.
The $64,000 question, does it all burn up?
I don’t think anyone can argue an internal combustion engine creates blow by. Likewise, I don’t think anyone can argue the need to have a ventilated crankcase either by a way-out-of-date “draft tube” or a more current PCV valve system. If you don’t agree with those two principles you should probably stop reading and find some books on internal combustion engines.
The $64,000 question… well actually its a $100 question (the cost of a catch can) is do you believe all of the contaminates pumped back into the intake manifold system are combusted with no ill effects or are they leaving an unwanted deposit on the intake manifold side of the engine?
6.1L with 36,000 miles (thanks to Spin2Win and the PNWLX Car Club Challenge MOPAR Team)
It would seem from pictures posted on the Challengertalk forum the answer is… maybe not. The pictures, one of which is shown at left, are reportedly from the intake manifold of a 6.1L Hemi with 36,000 miles on the clock.
The intake manifold clearly shows unburned oil deposits on the intake runners. I noted, with some interest, the clean portions barely visible and wondered if this was caused by the fuel injectors, which could lead to a whole atomization argument in fuel injection systems vs carburated atomization. I’ll save that one for later.
It is interesting to note there is NO buildup of sludgy looking deposits on the oiled portion of the motor (shown in the lower part of the picture).
From that image alone I think it is safe to assume the PCV system is working and it is doing exactly what it is designed to do, which is pump oil vapor and contaminates from the crankcase to the fuel induction system. Its been said a picture is worth a thousand words. In this case the thousand words can be reduced to a few. It appears the oil from the PCV system is coating the inside of the intake manifold and leaving a buildup.
In my mind the question boils down to two things:
1. Would a catch can have resulted in a cleaner intake than the one pictured?
2. Does the oil coating in the intake manifold have an adverse effect on performance.
I’ll let you come to your own conclusions on those two questions.
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