Busting my own MYTH - oil catch can

[ShadowsPapa]

(21 JTR, 96k kms TL, second engine has just over 60k kms)
So, I’ve had a long standing belief that if I couldn’t “feel” air blowing out the oil fill cap while engine running, my rings were seated perfectly and didn’t have blowby and thereby, didn’t need an oil catch can. WRONG!

You still don't really "need" one.
Take that amount of oil. That's 1,000 miles, correct?
Now, divide that into 1,000 equal parts. That will be the amount of oil getting in through the PCV system in 1 mile.
Figure the average road speed, average RPM and so on and do the math - each 2 RPM is a single power stroke and intake stroke.
So for 1,000 RPM, you have 500 intake strokes.
Divide by 6 for a V6 and that way you can figure how small an amount of oil is getting in for each intake stroke for each cylinder in your case.
Since in your case, it appears to be pure oil, unlike what some others show and freak out over, it can technically go right back into the crankcase.
Hey, that's what the cyclonic separator at the engines PCV does - catches the oil and drops it back into the engine. The catch can is just another cyclonic separator down the line a bit from the factory one. It's oil from your own crankcase.

Think of the millions of 3.6 engines out there running fine without such a device.
And of those, over half are the PUG engines, like we have in the JT.

Imagine those that are "consuming" 1/2 or more quarts of oil between oil changes - without any issue.
Oddly when one installs a catch can, if they see any oil, it's assumed that it is horrible and it proves they needed one. I still think you have the term "blowby" misinterpreted. There is ALWAYS some blowby. Always. That is unless you use torque plates and concentrate on finishing each cylinder to exactly perfectly round, and use gapless rings and so on - even then, it's next to impossible to hit 0 blowby.
Look at what the labs suggest you do before pulling a sample - drive it, get the fuel burned out of the crankcase and so on, then pull your sample. That's because there's always blowby in production engines - always.
Yes, your engine has blowby, mine has blowby, every vehicle I've ever owned, new or used, has blowby. It's combustion gases getting past the rings.
Is it excessive blowby? Only if the PCV system can't keep the crankcase evacuated and pressure builds.
Do you have blowby in your engine? Of course! We all do!
Is it excessive? No.
The amount of oil in a catch can is not indicative of blowby being excessive.
It only means that for some reason, the stock system isn't "filtering out" the oil if the amount in the catch can is more than what you are seeing.
In short, you don't have engine problems, you don't have excessive blowby, that oil isn't meaning anything, really.

I see the catch can as a way to make up for other conditions, especially on heavily modified vehicles, and for Jeeps or other vehicles used hard off road at severe angles, and load conditions.
Not a necessity for even a large portion of our engines.
Doesn't hurt anything to have one, but in this case, it's not proving there's a problem, either.

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[willys 41]

You still don't really "need" one.
Take that amount of oil. That's 1,000 miles, correct?
Now, divide that into 1,000 equal parts. That will be the amount of oil getting in through the PCV system in 1 mile.
Figure the average road speed, average RPM and so on and do the math - each 2 RPM is a single power stroke and intake stroke.
So for 1,000 RPM, you have 500 intake strokes.
Divide by 6 for a V6 and that way you can figure how small an amount of oil is getting in for each intake stroke for each cylinder in your case.
Since in your case, it appears to be pure oil, unlike what some others show and freak out over, it can technically go right back into the crankcase.
Hey, that's what the cyclonic separator at the engines PCV does - catches the oil and drops it back into the engine. The catch can is just another cyclonic separator down the line a bit from the factory one. It's oil from your own crankcase.

Think of the millions of 3.6 engines out there running fine without such a device.
And of those, over half are the PUG engines, like we have in the JT.

Imagine those that are "consuming" 1/2 or more quarts of oil between oil changes - without any issue.
Oddly when one installs a catch can, if they see any oil, it's assumed that it is horrible and it proves they needed one. I still think you have the term "blowby" misinterpreted. There is ALWAYS some blowby. Always. That is unless you use torque plates and concentrate on finishing each cylinder to exactly perfectly round, and use gapless rings and so on - even then, it's next to impossible to hit 0 blowby.
Look at what the labs suggest you do before pulling a sample - drive it, get the fuel burned out of the crankcase and so on, then pull your sample. That's because there's always blowby in production engines - always.
Yes, your engine has blowby, mine has blowby, every vehicle I've ever owned, new or used, has blowby. It's combustion gases getting past the rings.
Is it excessive blowby? Only if the PCV system can't keep the crankcase evacuated and pressure builds.
Do you have blowby in your engine? Of course! We all do!
Is it excessive? No.
The amount of oil in a catch can is not indicative of blowby being excessive.
It only means that for some reason, the stock system isn't "filtering out" the oil if the amount in the catch can is more than what you are seeing.
In short, you don't have engine problems, you don't have excessive blowby, that oil isn't meaning anything, really.

I see the catch can as a way to make up for other conditions, especially on heavily modified vehicles, and for Jeeps or other vehicles used hard off road at severe angles, and load conditions.
Not a necessity for even a large portion of our engines.
Doesn't hurt anything to have one, but in this case, it's not proving there's a problem, either.

Take the oil from a catch can and have it analyzed.
It will be full of carbon, water and fuel.
Not something I want back in my crank case or in my intake air.

 

[ShadowsPapa]

Take the oil from a catch can and have it analyzed.
It will be full of carbon, water and fuel.
Not something I want back in my crank case or in my intake air.

Why would it be full of carbon? Please explain how carbon is introduced into the PCV system air stream if it's not in the crankcase.
The oil in a catch can came from your crankcase via the PCV line to the intake.
It's what the PCV didn't filter out and drop right back into the crankcase via the head.
If there's carbon in it, then there's also carbon in the oil in the crankcase since that's exactly where it came from.

Why would it have fuel? It came out of the crankcase - it's the exact same oil that's flying around in the air in the crankcase. When you get the engine up to full operating temperature for long enough, fuel contamination is "burned out" as is water.
If there's fuel in there, then you make only short drives and you have fuel contamination in your crankcase already/anyway.

There is no reason that oil would have more carbon or fuel than the oil that's in the crankcase because what's in the catch can came directly from the crankcase.

The catch can is simply a second cyclonic device that catches oil out of the air stream headed into the intake manifold, it's catching oil from your crankcase.
You should only show fuel or water if you only make short drives. Contamination in a catch can indicates contamination of the oil in the engine.

Since my oil shows no contamination of fuel, water, etc. - then the oil coming out of the PCV, which is directly from the air in the crankcase, should also show no contamination.
If it would - how would carbon get in there??

 

[willys 41]

Why would it be full of carbon? Please explain how carbon is introduced into the PCV system air stream if it's not in the crankcase.
The oil in a catch can came from your crankcase via the PCV line to the intake.
It's what the PCV didn't filter out and drop right back into the crankcase via the head.
If there's carbon in it, then there's also carbon in the oil in the crankcase since that's exactly where it came from.

Why would it have fuel? It came out of the crankcase - it's the exact same oil that's flying around in the air in the crankcase. When you get the engine up to full operating temperature for long enough, fuel contamination is "burned out" as is water.
If there's fuel in there, then you make only short drives and you have fuel contamination in your crankcase already/anyway.

There is no reason that oil would have more carbon or fuel than the oil that's in the crankcase because what's in the catch can came directly from the crankcase.

The catch can is simply a second cyclonic device that catches oil out of the air stream headed into the intake manifold, it's catching oil from your crankcase.
You should only show fuel or water if you only make short drives. Contamination in a catch can indicates contamination of the oil in the engine.

Since my oil shows no contamination of fuel, water, etc. - then the oil coming out of the PCV, which is directly from the air in the crankcase, should also show no contamination.
If it would - how would carbon get in there??

It's called blow-by.

 

[Lunentucker]

I've been saving my catch collections. If I get curious enough I might send them in for analysis.

 

[Maximus Gladius]

I think for experimental, learning, data points, prove or disprove learned reasoning or speculation ….and for shits and giggles, I’ll have the oil in the catch can analyzed. It only takes a day to find out my results.

I would, however, like to know about the difference in crankcase air pressure, either (-) or (+) that’s felt and heard by these parts on the engine that either suck or blow that isn’t consistent and can’t explain, to me, the pressure in the crank case.

1. The 6 cylinders are sucking huge amounts of air and divides it to the exhaust and a bit or a lot down the rings (Blowby).
2. PCV is sending the crank case air pressure to the intake manifold. This air is either being pulled or being pushed, ..I think the intake is pulling it out to be reburned and out the exhaust and back through the rings in various amounts according to how well rings are seated.
3. I can’t feel this same amount of PCV air coming out the oil cap with engine running.
4. the MUA (make up fresh air) isn’t sucking from the intake tube like the PCV is blowing

So where is all this air coming from that’s in the crank case?

 

[willys 41]

I've been saving my catch collections. If I get curious enough I might send them in for analysis.

I hope you do get it analysed.
We need to put this to rest and show others what they are sending back though there engines.
Mine is a dirty oil put I have seen others with nasty stuff in there catch can.

 

[willys 41]

I think for experimental, learning, data points, prove or disprove learned reasoning or speculation ….and for shits and giggles, I’ll have the oil in the catch can analyzed. It only takes a day to find out my results.

I would, however, like to know about the difference in crankcase air pressure, either (-) or (+) that’s felt and heard by these parts on the engine that either suck or blow that isn’t consistent and can’t explain, to me, the pressure in the crank case.

1. The 6 cylinders are sucking huge amounts of air and divides it to the exhaust and a bit or a lot down the rings (Blowby).
2. PCV is sending the crank case air pressure to the intake manifold. This air is either being pulled or being pushed, ..I think the intake is pulling it out to be reburned and out the exhaust and back through the rings in various amounts according to how well rings are seated.
3. I can’t feel this same amount of PCV air coming out the oil cap with engine running.
4. the MUA (make up fresh air) isn’t sucking from the intake tube like the PCV is blowing

So where is all this air coming from that’s in the crank case?

Past the piston rings. All ICE engines have blow-by past the piston rings. Some more than others.
These gasses have fuel and carbon.
The water gets in the oil when you turn off the motor and as it cools down will draw moist air in and is more prevalent in damp humid areas.

 

[ShadowsPapa]

It's called blow-by.

Of course, DUH. But why would there be carbon there and not in the oil in the crankcase or that the stock PCV cyclonic filter catches and drops back into the crankcase?

I've been saving my catch collections. If I get curious enough I might send them in for analysis.

There's going to be a lot of variables based on the fuel you use (pure gas vs. any ethanol blends), long drives vs. short drives and so on. If you look at how my oil samples are coming back, I typically drive enough to "burn off" fuel contaminations and so on. There's no water or fuel in my oil samples.
Since the oil caught in a catch can is what's tossed about in the crankcase, it should be similar to sampling the water tossed off the end of a rotating sprinkler head vs. what's in the hose feeding the sprinkler.
I can see more water being in there because it never gets burned out and many catch cans experience a lot of heating and cooling cycles, leading to water in the can.
But oil looking like what Kevin's looks like - that looks like he pulled it right out of the crankcase (well, he did, technically speaking)

Blowby gases are mixed in with any oil in the crankcase. But the fuel contamination is burned out by getting the engine hot enough long enough. That doesn't happen with oil caught in a catch can, so I'd expect some fuel contamination in there - it's isolated from the crankcase oil and not heated to burn it off - like moisture, it's going to stay there or even be made worse in the catch can.
But carbon?

I think for experimental, learning, data points, prove or disprove learned reasoning or speculation ….and for shits and giggles, I’ll have the oil in the catch can analyzed. It only takes a day to find out my results.

I would, however, like to know about the difference in crankcase air pressure, either (-) or (+) that’s felt and heard by these parts on the engine that either suck or blow that isn’t consistent and can’t explain, to me, the pressure in the crank case.

1. The 6 cylinders are sucking huge amounts of air and divides it to the exhaust and a bit or a lot down the rings (Blowby).
2. PCV is sending the crank case air pressure to the intake manifold. This air is either being pulled or being pushed, ..I think the intake is pulling it out to be reburned and out the exhaust and back through the rings in various amounts according to how well rings are seated.
3. I can’t feel this same amount of PCV air coming out the oil cap with engine running.
4. the MUA (make up fresh air) isn’t sucking from the intake tube like the PCV is blowing

So where is all this air coming from that’s in the crank case?

There is ALWAYS blowby - meaning "some" combustion gases always get past the rings in production engines.
PCV is almost always a "suction" thing meaning the intake pulls air from the crankcase. That's why it's called "positive crankcase ventilation" - it should pull a vacuum, the intake should remove more air than blowby produces. The makeup air handles that.
It gets really complicated under WOT conditions, heavy loads, high vacuum conditions and so on. The PCV varies the amount of air pulled from the crankcase based on engine load or intake vacuum conditions.
Downshift and let off the throttle, you have extreme intake vacuum but low load and low blowby so the high vacuum of the intake closes down the PCV valve to reduce the amount of air pulled from the crankcase.
WOT, low intake vacuum, heavy load, the PCV opens up

 

[ShadowsPapa]

Past the piston rings. All ICE engines have blow-by past the piston rings. Some more than others.
These gasses have fuel and carbon.
The water gets in the oil when you turn off the motor and as it cools down will draw moist air in and is more prevalent in damp humid areas.

The water/moisture in the crankcase is actually a byproduct of combustion. It's in there via combustion gases - the blowby.
Since it's only open through the PCV and makeup air hose, it's not a great place for the moisture to get in. You'll find water and acids in the crankcase even in places like AZ.
I've got a couple of steel oil pans with nice holes in them due to the lack of oil changes, and short drives. And they didn't come from humid states.

Burning 1 gallon of gas produces 1 gallon of water! That means, there's a whole lot of water in blowby.

 

[willys 41]

Of course, DUH. But why would there be carbon there and not in the oil in the crankcase or that the stock PCV cyclonic filter catches and drops back into the crankcase?



There's going to be a lot of variables based on the fuel you use (pure gas vs. any ethanol blends), long drives vs. short drives and so on. If you look at how my oil samples are coming back, I typically drive enough to "burn off" fuel contaminations and so on. There's no water or fuel in my oil samples.
Since the oil caught in a catch can is what's tossed about in the crankcase, it should be similar to sampling the water tossed off the end of a rotating sprinkler head vs. what's in the hose feeding the sprinkler.
I can see more water being in there because it never gets burned out and many catch cans experience a lot of heating and cooling cycles, leading to water in the can.
But oil looking like what Kevin's looks like - that looks like he pulled it right out of the crankcase (well, he did, technically speaking)

Blowby gases are mixed in with any oil in the crankcase. But the fuel contamination is burned out by getting the engine hot enough long enough. That doesn't happen with oil caught in a catch can, so I'd expect some fuel contamination in there - it's isolated from the crankcase oil and not heated to burn it off - like moisture, it's going to stay there or even be made worse in the catch can.
But carbon?



There is ALWAYS blowby - meaning "some" combustion gases always get past the rings in production engines.
PCV is almost always a "suction" thing meaning the intake pulls air from the crankcase. That's why it's called "positive crankcase ventilation" - it should pull a vacuum, the intake should remove more air than blowby produces. The makeup air handles that.
It gets really complicated under WOT conditions, heavy loads, high vacuum conditions and so on. The PCV varies the amount of air pulled from the crankcase based on engine load or intake vacuum conditions.
Downshift and let off the throttle, you have extreme intake vacuum but low load and low blowby so the high vacuum of the intake closes down the PCV valve to reduce the amount of air pulled from the crankcase.
WOT, low intake vacuum, heavy load, the PCV opens up

You are assuming that every one drive like you.
Some drive hard. Some baby there jeep and some drive very short distances.

 

[ShadowsPapa]

You are assuming that every one drive like you.
Some drive hard. Some baby there jeep and some drive very short distances.

Seriously?
You have no clue. I made it very clear all of the variables, and modified vs. not and so on.
One just can't use logic at all with you.
I've been dealing with engines and vehicles since before half of the members here were born, and I know ALL of the variables, and how these things get used - or not used.
Even I don't have a "standard driving habit" or situation. Some days it gets beat on, some days it goes to the doctor and back, a casual drive.
I've run my JTs up way past lose your license speeds, and I beat on it plowing snow.

I also know the science of combustion, the chemistry, and the ways PCV systems work, valving, testing, and so on.

I am assuming nothing because every vehicle I've ever worked on has been used/driven in different ways. Some babied, some ridden hard and put away wet.

Looks to me like you are the one assuming - like saying the moisture is there because it comes from the atmosphere?

I try to back things with science, reality, and experience of decades, and get back this?

Once again, I'm done. I try to give real answers and true explanations - but no............

 

[RodRecket]

Catch can threads (for this forum) are right up there with Which oil should I run? and Which air intake will give me the most HP?

Lots of debate, but no real gain.

 

[hjdca]

Here is the bottom line. Everybody that has put a catch can on these motors gets oil in the catch can. The only question is whether pushing that oil back in the intake is harmful or not. We have a TSB that is telling us that, depending on the amount of oil, it could be harmful and cause carbon build up, which in turn causes pinging. So, it depends on each individual motor. If an owner wants to be safe, and not take a chance that too much oil is going back into the intake, then, it makes perfect sense that they add a catch can.... ie. somebody that tows in the mountains, and is at high rpm more than others could be at more risk than someone that drives on flat land at low rpm.... In my case, I am in the mountains, I am always on inclines, I was getting an EGR error, and I found carbon on my EGR valve, and I decided to add a catch can.

 

[Maximus Gladius]

Catch can threads (for this forum) are right up there with Which oil should I run? and Which air intake will give me the most HP?

Lots of debate, but no real gain.

For me, I’m happier knowing I’m not burning this oil and fouling plugs, sensors and the catalytic converters.

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