Oil out breather - unlikely that it’s blowby

[Ragsf15e]

I’ll add pictures later this week when I go clean the belly, but here’s the story…

TSMOH - 220hours, 3 years 

Last Annual - April 2025

When I got my 252 in January, one of the first things I noticed was how much oil was on the belly after a flight. It wasn’t actually using much according to the dipstick, but the spray was from the breather tube all the way to the tip of the tail along the belly.  At first, I thought it was blow by, but then I realized the oil was still looking new after 25 hours since oil change and the compressions were all good at the annual in the 60s. Then I realized the oil cap was a little loose so I changed the O-ring on it and it seats nicely now. Finally, I thought I solved the problem when I realized that both the breather and the return line from the oil separator were routed down underneath the cowling to exit the engine compartment. Yes, the oil return line was not hooked up to return oil to the engine. My mechanic fixed that at the annual and cleaned out the oil separator.  
 

The next five hours of flying had a small amount of oil out the breather which I considered normal. Then this weekend I flew two ~3 hour XC legs and again I have a lot of oil out the breather. I would say it’s not as much as before the annual, but it goes from the breather tube all the way back below the avionics compartment.  The engine only uses about 1 quart every 10 hours.  When I left on my cross country this weekend, there were 6.5 quarts of oil in the engine. After the first three hour leg I checked and it still had more than six, so I didn’t add any. I’ve been trying to keep it right around six because lots of people say it will blow out more than that. Does anyone keep it below six?

From my reading, it seems that it could have something to do with a prop shaft seal or maybe the dipstick isn’t sealing right or maybe the breather or oil separator are clogged somehow? Or maybe pointing the wrong way? I have no idea how we should go about troubleshooting this. I really do not think it’s blow by from a cylinder.

My breather is bent near the exit and ziptied there to point directly aft.  Is that how others are?  My F model breather opened straight down.

I feel like I'm grasping at straws… any suggestions?

Thanks,

Drew

 

Edited May 19 by Ragsf15e

[N201MKTurbo]

Do a compression check and listen at the oil fill.

[takair]

Where does the oil return line from the separator go now?  There seems to be a science to getting it right too.  If the routing allows oil to build up it becomes ineffective.  

[Ragsf15e]

47 minutes ago, takair said:

Where does the oil return line from the separator go now?  There seems to be a science to getting it right too.  If the routing allows oil to build up it becomes ineffective.  

There is a small tube on the bottom of the oil filler tube for the return line.  I’ve actually been thinking about this because routing that line has to go through the back baffle (filler tube is forward of the baffle, oil sep is aft), and my return line is in an inverted U shape right now stuffed through a hole with some other lines (maybe ignition wires?).  Almost has a “P” trap in it…

[Z W]

Mine used to blow out anything over 6 quarts. I would let it drop to near 5, then fill to 6. Oil pressures and temps were fine this way and the internet said it was OK to do. My POH says "Oil Capacity Minimum for Flight - 5 qts."

After cleaning out the air/oil separator and lines and replacing the oil cap gasket, that improved, and now it holds 8 pretty well, but seems to do best between 6 and 7, so I add a quart at 6, about once every 10 hours of flight.

All measurements here are described after the engine has sat off for 30+ minutes. If you take a reading right after landing you will be 1 quart lower than described.

[Ragsf15e]

Here’s what the belly looks like, how the breather is oriented and the oil separator return line looks.

[jlunseth]

Well, this is a story I have told before so those who have read it, bear with me. I got to do an exciting power-back descent and landing while overflying Canada once. It turned out a tiny but long sliver of plastic, sort of like one of those fingers on an oil bottle cap, had been swimming around in the oil, went into the engine quick drain and was too long to pass through the seal. I noticed a drip on the nose tire so had taken the plane in to the shop immediately before the trip and was told not to worry, quick drains just leak a little. We blew most of the oil out in a couple of hours. The pressure fell to 14 psi (normally around 38) and then when I powered the engine back to idle and tipped the nose down it fell to 6 psi. The landing worked out. Before this flight I was plagued with oil on the belly and it would be variable, some trips the plane would use alot of oil, like a quart an hour, some trips not much. Like you I was trying to chase problems with the air/oil separator or other leaks in the engine. Now, at annual, I have the mechanic remove the quick drain entirely when the oil is changed so anything swimming in the oil can exit through the big hole and ever few years I have a new seal kit installed in the quick drain. Its been more than ten years and two engines since then, and my 231 uses about a quart between oil changes, such a small usage that I can start with 7 qts and wind up at 6 at oil change time.

PS power off descent from 19k with strong tailwinds blowing us over Lake Huron, about 7k of the descent was IMC, all worked out with a perfect landing. Getting out of Canada and back to the US without passports or EAPIS credentials was a problem.

Edited Thursday at 01:44 PM by jlunseth

[geoffb]

Rags,

I'm having the same issue on my -MB ever since my last annual.  It's definitely coming out the breather.  I don't think my separator has ever been cleaned, so I plan to remove, clean and go over the return line routing this weekend when I change oil.

Thinking about this a bit, the crankcase probably always has a positive pressure due to blowby or we wouldn't need the vent and separator  So does oil return to the sump from the breather only after shutdown?  Anything that prevented the separator from draining would allow oil to remain in the separator.

 

Geoff

[N201MKTurbo]

I do annuals on a C310 with two big Continentals. Last year we cleaned both separators. I don’t think they had ever been cleaned. The screens were pretty much plugged and there was gunk in the bottom akin to asphalt. We removed them from the engine to cleaned them. I don’t think any oil was draining back, it was just building up till it went out the big hose.

[Ragsf15e]

15 hours ago, geoffb said:

Rags,

I'm having the same issue on my -MB ever since my last annual.  It's definitely coming out the breather.  I don't think my separator has ever been cleaned, so I plan to remove, clean and go over the return line routing this weekend when I change oil.

Thinking about this a bit, the crankcase probably always has a positive pressure due to blowby or we wouldn't need the vent and separator  So does oil return to the sump from the breather only after shutdown?  Anything that prevented the separator from draining would allow oil to remain in the separator.

 

Geoff

If you take the time to remove/clean the separator, I suggest doing the whole breather tube “system” from the engine connection to the outlet.  We cleaned my separator but I am going to have my IA help me clean out the whole thing as the tubes apparently gunk up too.  Also, I agree with your speculation about the drain… I wonder how it actually drains?  We are going to route my drain line much better.

@N201MKTurbo do the separators you cleaned come apart?  The on on the 252 is sealed, so we just ran solvent through it for a few minutes and called it good.  I wish I could take it apart.

[N201MKTurbo]

1 hour ago, Ragsf15e said:

If you take the time to remove/clean the separator, I suggest doing the whole breather tube “system” from the engine connection to the outlet.  We cleaned my separator but I am going to have my IA help me clean out the whole thing as the tubes apparently gunk up too.  Also, I agree with your speculation about the drain… I wonder how it actually drains?  We are going to route my drain line much better.

@N201MKTurbo do the separators you cleaned come apart?  The on on the 252 is sealed, so we just ran solvent through it for a few minutes and called it good.  I wish I could take it apart.

Yes, they had a thumbscrew on top so you could clean the screen. In our case, the body was so gunked up, oil couldn’t flow out the return line.

[Pinecone]

I had a similar issue.  FBO removed the line from the right front of the engine to the oil separator and the oil separator itself.  They flushed with both solvent and everything was back to normal