Still confused. Do you mean that at expressway speed the blow-by gasses would go back into the engine in two separate paths, some through the PCV valve and throttle body, and some, in the reverse direction shown in the “streetrod” diagram, into the engine through the air cleaner?
Yes. Whenever blowby exceeds the capacity of the PCV valve the excess is drawn into the intake plenum at the air filter.
Have you seen blowby escaping out the oil fill cap when it is removed from a running engine? It’s obviously more than the PCV can extract. So where does it go when the oil cap is re-installed?
I’ve never done that experiment. I have noticed a little oil-fouling of my truck’s engine air filter. I thought that was hot gasses rising from the engine’s innards after the engine was turned off.
When I used to change air filters in the '70s, there was a small filter in the air filter housing at the end of the PCV hose connected to the valve cover. That filter was always really dirty, and the side of the air filter near it was always dirty, too. This supports @Rod_Knox that air is typically going from the engine into the air filter housing, not the reverse.
That small filter at the end of the vent hose makes it obvious that crankcase blow by was intended to be drawn into the intake via that route from the beginning regardless what the technical drawings show @texases.
And as for the catch cans, I can’t see how those can be cost effective. They can’t recover a usable amount of oil and the fumes continue on to the throttle where they condense and accumulate despite the catch can.
Well, normally, the small coarse filter was outside of the normal cylindrical air filter. So vapors going “backwards” had to go through both before being ingested. My cars always had the coarse filter soaked in oil but they were old, well used engines. It was just a breather hose with no valve in it. So some amount of back flow was probably inevitable. The more modern PCV systems have the spring opening the valve so that as vacuum decreases, the valve opens more to compensate. On a new engine, they probably work OK but as it gets worn, probably not quite as effective.
The PCV valve is on the second hose, coming from the valve cover to the carb. I bet your car had both hoses.
You talkin’ to me? Yeah, it had a breather hose with a plastic elbow stuck in a grommet on the valve cover. Trust me, I’ve had cars with PCV valves and those without. Not my first rodeo…
I know you know your cars, I’m just confused - are you saying you had cars with the hose to the air cleaner, with that small extra filter, but no second hose with a PCV valve? I haven’t seen that combination.
You’re probably right. That’s the way I distinctly recall having some cars back in the day but coming up short looking online for evidence to support it. The time I’m thinking of was right around the transition period. Come to think of it, none of my cars were new back then and I know a lot of hot rodders simply tossed those valves anyway. The restriction of the tubing going into the carb body was enough to limit the air bleed and not cause issues. Looking more and more likely old age creeping up on me…
Yeah, I remember seeing plenty of those plugged off with a bolt, etc…
That’s the way my 1975 Civic was.
Did that have the CVCC engine? Don’t know if that had anything to do with it, of course…
Funny thing is the British crankcase vent was called an ‘anti-backfire valve’ and the American PCV valve has mostly that same function.
Funny that @Rod_Knox mentioned the British crankcase vent, as I just ran across an online article on the evolution and operation of PCV systems, both British and American.
Yeah, CVCC.
A friend had non-CVCC and it was the same IIRC.
Also had manual choke and points.
More like 1965 than 1975.
I don’t understand. Isn’t the whole point of a catch is not obtain a usable amount of oil that can be like put back into the crank case, but to remove the oil/fuel from the PCV system to be recycled? It’s keeps the substance from going into the intake. This substance contains oil, if it goes into the intake to get burned, doesn’t burned oil lead to carbon buildup, leading to carbon buildup on the intake/outtake valves, upper cylinder heads, and combustion chambers? It’s supposed to help stop the buildup of carbon by keeping oil from being burned.
Isn’t the whole point to to keep it from getting burned.
I have ran this experiment in my car. Maybe about 1000 miles, and I have a yellow sludge in the can. It’s gross stuff. It’s chunky in some areas, but oily on top. It’s like a chunky form of snot and a dark yellow. It looks sickening. If it wasn’t for this catch can this stuff would have gone through the intake in air form and have been burned. So I can say they do what they are supposed to. Keep the fluids from blowby from going back into the intake.
I put one on my 2018 Hyundai Accent with gdi engine.
Wasn’t too expensive and I figured it was worth trying. I was surprised at the amount of fluid in 5000 miles.
Some say it’s mostly condensation from hot gases going thru relatively cooler lines. Maybe so but there’s some oily crud in there too. Looks like chocolate milk.
And no, the engine doesn’t burn any oil.
2 screws and some hose clamps returns thing to stock in case it needs to go in for warranty work.
Got 20k on it now and it hasn’t needed any yet.
Pretty nice little commuter car
I thought blow by gases were a oil/fuel mixture in mist form? Oil catch can condenses the mist into liquid form. Wouldn’t burning this oily crud result in carbon build up?