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Forged 2007 STi Hawk,
I've had a Mocal catch can for several years now, and never seemed to "catch" much, just oily vapour at most.
Considering it's never caught much, I think I may have a serious problem.
It's connected to the breathers on the rocker covers that all, as far as I can see. The PCV valve is blocked off and not connected to anything.
Today I took the catch can cap off, and to my horror there is oil at the bottom of it, not sure how much but it doesn't show on the site tube. The underside of the vented cap is also milky.
Have I perhaps got excessive blow by? If I take off the engine oil filler cap and place it loose on the oil filler neck, it stays there and dances around, it doesn't blow off, no smoke.
Maybe it's all normal? But for years it never really caught anything other than a residue at most.
The passenger side breather port on the catch can was milky and gunky, the other port looked pretty clear.
It doesn't drain back to sump, the drain pug it underneath the can.
How could I get coolant into the ports if it's coolant I'm seeing, perhaps it's just condensation?
It could be the can just doing it's job, but I don't know if this oil I can see has just got in there from the 80 mile drive I've just done, or over the course of months, perhaps the oil is just a bit high....cold level it's up to the notch on the dipstick, so a lot of oily vapours moving around?
I've drained the can so will monitor.... pics below. The oil is opaque and brown looking, about 70-80ml of oil at a guess.
This is the can :
It’s not coolant in your catch tank, it’s just condensation. Have a look inside any catch can or AOS and you will see the same. It’s hot blowby gasses hitting the cold catch can walls that creates this. If you look inside breather pipes on any vehicle you will see the same. An AOS returns this to the sump where the condensation burns back off and you are left with clean oil.
I'll check, but it's usually clear - a bit grubby, but clear if that makes sense. Oil only changed 2k miles ago.
I did your tip when we spoke on the phone with the dancing of the oil filler cap.
Fingers crossed....
But I've had this catch can for many years, never caught anything, why now I wonder...
The car does do fewer journeys now I guess as I don't daily it anymore, but even so, I've not seen this before!
I hope I'm just suffering Subaru paranoia....it goes hand in hand with 2.5 ownership lol
Edit, yes oil on the dipstick is fine, a bit grubby but pretty clean and not the slightest bit murky like the catch can oil.
Is a rebuild worst case scenario on this do you think? Temps are fine and car runs well....
Last edited by Hawkeye D; Jan 29, 2023 at 02:20 PM.
It’s not coolant in your catch tank, it’s just condensation. Have a look inside any catch can or AOS and you will see the same. It’s hot blowby gasses hitting the cold catch can walls that creates this. If you look inside breather pipes on any vehicle you will see the same. An AOS returns this to the sump where the condensation burns back off and you are left with clean oil.
It's a lot of blowby gasses then, I see 70-80ml of catch can oil as a serious issue, but maybe I'm just seeing the can do it's job.
I'm just stressing because the can has never seen volumes anything even close to what I found today.
It's got a vent to atmosphere cap on it.
Also perhaps contributing is a higher than my normal oil level, nothing too much, it's just below the notch on cold level.
Last edited by Hawkeye D; Jan 29, 2023 at 02:21 PM.
It's a lot of blowby gasses then, I see 70-80ml of catch can oil as a serious issue, but maybe I'm just seeing the can do it's job.
I'm just stressing because the can has never seen volumes anything even close to what I found today.
It's got a vent to atmosphere cap on it.
Also perhaps contributing is a higher than my normal oil level, nothing too much, it's just below the notch on cold level.
Its oil too, but it’s not coolant mixing with the oil, it’s condensation.
But yes, you are effectively seeing the can doing what it should. Any number of reasons why you have seen an increase in amount of oil caught. Are you venting from the head vents only?
Yes, the only pipes going to the two ports on the catch can are the rocker cover breathers, that's it.
How does condensation build up?
Some history from a couple of years ago (20k miles ago)....
I did have a cracked pick up pipe which caused a flickering of the oil light on start up for about 15 seconds sometimes. We didn't know the cause but had a 12mm oil pump fitted (rather than a standard 11mm) which didn't cure it, then was found out to be the pick up so that was replaced too.
Would a 12mm oil pump cause it possibly? I never knew why a 12mm oil pump was fitted and not 11mm like what came out.....It's the only hard thing oil related that I can think of that may push around more oil than perhaps needed.
Yes, the only pipes going to the two ports on the catch can are the rocker cover breathers, that's it.
How does condensation build up?
Some history from a couple of years ago (20k miles ago)....
I did have a cracked pick up pipe which caused a flickering of the oil light on start up for about 15 seconds sometimes. We didn't know the cause but had a 12mm oil pump fitted (rather than a standard 11mm) which didn't cure it, then was found out to be the pick up so that was replaced too.
Would a 12mm oil pump cause it possibly? I never knew why a 12mm oil pump was fitted and not 11mm like what came out.....It's the only hard thing oil related that I can think of that may push around more oil than perhaps needed.
A catch can that’s only piped up to the head vents isn’t going to do much, it’s the crank case port that’ll see most of the engine blowby, so really you need to be plumbing into there as well as the head vents. If you had done that you would see much more oil in your catch can. As it is you are seeing oil from slosh as the oil moves to the heads under cornering. It’s really worth investing in a decent AOS that returns to sump, then it’s just fit and forget. The main purpose of a catch can or AOS is to remove the risk of octane dilution via oil in the inlet tract. With the crankcase vent still looping back into the inlet you aren’t really curing anything.
condensation builds up from heat. Put hot steam into a cold glass and water droplets will form as the water molecules fall out of suspension. This is what’s happening in your catch can. The can itself will be colder than the hot engine blowby, hence moisture forms. Some AOS’s have pre-heated water jackets to warm the sides of the AOS to prevent this from happening but it’s more of a gimmick as when the oil returns to the hot sump, the water droplets turn back into steam and leave the oil clear.
A 12mm oil pump is overkill really. It should only been used on a dual AVCS setup that’s running something like an external oil cooler. An 11mm pump is more that man enough for most, however I can’t see that being a problem. As I said, oil getting up into the heads is from slosh going round corners and what you are seeing is completely normal. This can be minimised with a baffled sump, but you will still get it to some degree. Some Subaru track cars have been known to fill a catch can in a couple of laps due to oil slosh.
I don’t see anything to be concerned with here, but you really do need to vent the crankcase vent into the catch can at least, that way you will be doing it properly. But keep an eye on it then as it’ll certainly fill quicker, or bite the bullet and go full AOS and never need to worry about it.
if you’ve suffered a cracked oil pickup and your engine is still running two years later I’d say you’ve dodged a bullet. If you really want to be belt and braces, get a compression and leak down test done. It’s simple and will give you an idea on your the condition of your rings.
A catch can that’s only piped up to the head vents isn’t going to do much, it’s the crank case port that’ll see most of the engine blowby, so really you need to be plumbing into there as well as the head vents. If you had done that you would see much more oil in your catch can. As it is you are seeing oil from slosh as the oil moves to the heads under cornering. It’s really worth investing in a decent AOS that returns to sump, then it’s just fit and forget. The main purpose of a catch can or AOS is to remove the risk of octane dilution via oil in the inlet tract. With the crankcase vent still looping back into the inlet you aren’t really curing anything.
condensation builds up from heat. Put hot steam into a cold glass and water droplets will form as the water molecules fall out of suspension. This is what’s happening in your catch can. The can itself will be colder than the hot engine blowby, hence moisture forms. Some AOS’s have pre-heated water jackets to warm the sides of the AOS to prevent this from happening but it’s more of a gimmick as when the oil returns to the hot sump, the water droplets turn back into steam and leave the oil clear.
A 12mm oil pump is overkill really. It should only been used on a dual AVCS setup that’s running something like an external oil cooler. An 11mm pump is more that man enough for most, however I can’t see that being a problem. As I said, oil getting up into the heads is from slosh going round corners and what you are seeing is completely normal. This can be minimised with a baffled sump, but you will still get it to some degree. Some Subaru track cars have been known to fill a catch can in a couple of laps due to oil slosh.
I don’t see anything to be concerned with here, but you really do need to vent the crankcase vent into the catch can at least, that way you will be doing it properly. But keep an eye on it then as it’ll certainly fill quicker, or bite the bullet and go full AOS and never need to worry about it.
if you’ve suffered a cracked oil pickup and your engine is still running two years later I’d say you’ve dodged a bullet. If you really want to be belt and braces, get a compression and leak down test done. It’s simple and will give you an idea on your the condition of your rings.
The voice of reason ! Always find that emulsion in the tubes and aos don't panick OP
Wow Danjo that's a very informative write up, and thank you very much for taking the time to do it.
Interesting about slosh, I've never been a twisty person, but a recent geo set up along with 4 new Michelin Pilot Sport 5 tyres, I have been hooning more than usual on the back roads, so that stacks up.
The inlet tract is all blocked off, apart from a narrow flexible hose which appears to go to the boost solenoid. Everything else has been blocked off for several years.
Wow Danjo that's a very informative write up, and thank you very much for taking the time to do it.
Interesting about slosh, I've never been a twisty person, but a recent geo set up along with 4 new Michelin Pilot Sport 5 tyres, I have been hooning more than usual on the back roads, so that stacks up.
The inlet tract is all blocked off, apart from a narrow flexible hose which appears to go to the boost solenoid. Everything else has been blocked off for several years.
No problem at all, I was about to ask if you’ve had new tyres, coilovers etc as slosh usually comes from harder than average cornering and you often see it when people are trying out new setups for grip.
Ok, had your PCV been deleted, and if so what is your crank case vent venting to? Normally you would delete the PCV on a forged build and run the crank case vent and head vents to an AOS and this would then vent to atmosphere, so yours must be venting somewhere. A good example of how it should be piped up is on the RCM website below:
The PCV has been deleted in this example and the crank vent runs straight into the can, as do the head vents. I’d be interested to see what has been done on yours as it’s important to ensure the blowby is getting out of the crank case without causing back pressure.
Here's a video I made last year when I was trying to work out my PCV set up....and I'm still not sure it's right? As said though, it has been set up like this for several years now.
Here's a video I made last year when I was trying to work out my PCV set up....and I'm still not sure it's right? As said though, it has been set up like this for several years now.
It’s hard to see from the video what’s happening with the PCV as it’s under the throttle body but it doesn’t appear that it’s going into the catch can. I can see one of the two outlets on the crank case t-piece is blocked off, it’s what’s happening with the other that’s the question. Typically it would go to the PCV valve that would allow pressure to be pulled back into the turbo inlet under normal driving conditions, this would then close off under boost so that boost pressure doesn’t pressurise the crank case. Looking at the video, both your PCV connections on your turbo inlet have been blanked off, so not sure how the pressure is exiting and to where? It would be worth getting the throttle body off to have a closer look.
It’s hard to see from the video what’s happening with the PCV as it’s under the throttle body but it doesn’t appear that it’s going into the catch can. I can see one of the two outlets on the crank case t-piece is blocked off, it’s what’s happening with the other that’s the question. Typically it would go to the PCV valve that would allow pressure to be pulled back into the turbo inlet under normal driving conditions, this would then close off under boost so that boost pressure doesn’t pressurise the crank case. Looking at the video, both your PCV connections on your turbo inlet have been blanked off, so not sure how the pressure is exiting and to where? It would be worth getting the throttle body off to have a closer look.
Would taking off my TMIC allow me to see a bit more?
Would taking off my TMIC allow me to see a bit more?
It’ll give you a slightly better view yes, althoigh it’s under the throttle body that’ll give you the best view. It’s bolted on with four long bolts, 10mm heads on them, and there’s a plastic gasket sandwiched in between the throttle body and inlet manifold that’s reusable, so easy enough to investigate further if needs be.
When Rebuilt my engine, I went for a Grimmspeed AOS. It drains into the oil filler so never needs emptying. Obviously a new engine shouldn't use oil but I added it for peace of mind. So far it's been trouble free.
The fitting instructions are rather poor but it's not difficult to figure out. I wanted to keep the original PCV and plumbed it in accordingly.
Oil in catch can
