Crankcase breather physics...

bigpoppa

bigpoppa

Well-known member
Doing a little troubleshooting/maintenance on the AT, in the process found this video...and am wondering if what this guy says is legit?

If so...it would explain the leaks...and oil burning...

 
HD's have had breathers for many years and still do today (just back to the inlet instead of directly to atmosphere)
IMO, this is where the "all Harley's leak oil stereotype came from"
 
Your OEM crankcase vent configuration should not lead to either oil consumption or leaks. There's a labyrinth flow path built into the crankcase that is designed to separate out substantially all oil mist and return it to the crankcase. The flow path is enough that if your piston rings are anywhere near decently seated (thus reducing leakage past piston rings to a normal range) there will not be significant back pressure. It's vented into the airbox, so any stuff that does need to be vented out of the crankcase, goes into the engine air intake to be burned, rather than leaking out as a liquid.

If it's consuming oil to any significant degree, or if oil is leaking out onto the ground and it isn't due to you lubricating the drive chain, something's wrong.
 
Brian P said:
Your OEM crankcase vent configuration should not lead to either oil consumption or leaks. There's a labyrinth flow path built into the crankcase that is designed to separate out substantially all oil mist and return it to the crankcase. The flow path is enough that if your piston rings are anywhere near decently seated (thus reducing leakage past piston rings to a normal range) there will not be significant back pressure. It's vented into the airbox, so any stuff that does need to be vented out of the crankcase, goes into the engine air intake to be burned, rather than leaking out as a liquid.

If it's consuming oil to any significant degree, or if oil is leaking out onto the ground and it isn't due to you lubricating the drive chain, something's wrong.
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I suppose not emptying the crankcase breather hoses wouldnt cause such issues?

Regardless...dealer has been contacted, warranty will be claimed
 
Overfilling can cause similar symptoms. If its puking oil into the airbox and on to the ground double check. Can't hurt ?
 
bigpoppa said:
I suppose not emptying the crankcase breather hoses wouldnt cause such issues?

Regardless...dealer has been contacted, warranty will be claimed
Click to expand...

If the system is stock then there shouldn't be anything to "not empty". There should be a direct downhill path back through the crankcase vent hose back into the crankcase. There's nothing to "empty" (there's nowhere to "not empty").
 
bigpoppa said:
I suppose not emptying the crankcase breather hoses wouldnt cause such issues?

Regardless...dealer has been contacted, warranty will be claimed
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That bad huh? Hope you get it fixed

This was something I was looking into when my HD died. I'm pretty sure it was sumping, and the breather system wasn't doing it's job (when I ripped it appart the breather gaskets were all done). Basically all cranks are under pressure, and need to relieve this someplace, if not other bad stuff happens (one of those things is sumping). The breather allows the pressure to release but reclaims misty oil/air and throws it back into the intake maniflold (typically), for environmental reasons. Or into some other capture system (catch can).

I found this video helpful. It's somewhat HD slanted but the overall theory is there with other non HD examples
 
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Check your airbox and see if there's oil in there. Should be bone dry, but some are prone to get more if you spend enough time on one wheel (either one). If you're riding sanely, and you have lots of oil in the airbox, it could be bad rings (or badly seated rings on a fresh motor) resulting in burning oil as well as blow-by over-pressurizing your crankcase and sending more than normal amounts of oil mist out the PCV. A common problem with improperly broken-in SXV's.
 
I see you mentioned leaks. Aside from poor sealant application, if the PCV valve or oil separation system is clogged or defective, that can also lead to crankcase over-pressurization which has to go somewhere - out the weakest seal.

Hopefully Honda will figure it out under your warranty.
 
Finally found a free service manual hidden away in the corner of the internet...
 
Is there another thread with more details?
How much oil are we talking about? A wet spot in a corner of the airbox is not normally an issue.
 
This is the leak in question:

Usually I find I have to top up the oil every 1000kms about 500ml or so.
 

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That doesn't look like a leak that would account for half a bottle of oil in 1000 kms, but if it is it might be an easy fix. Looks like it's coming from the round cap which looks like an oil filter cover that's usually sealed with a simple o-ring. If care isn't taken when reassembling, it can get caught and chewed up by the threads.
 
Relax said:
That doesn't look like a leak that would account for half a bottle of oil in 1000 kms, but if it is it might be an easy fix. Looks like it's coming from the round cap which looks like an oil filter cover that's usually sealed with a simple o-ring. If care isn't taken when reassembling, it can get caught and chewed up by the threads.
Click to expand...
yes, I believe that is the DCT oil filter...
 
Brian P said:
If the system is stock then there shouldn't be anything to "not empty". There should be a direct downhill path back through the crankcase vent hose back into the crankcase. There's nothing to "empty" (there's nowhere to "not empty").
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you were correct, checked the service manual, and what the internet forums called a crankcase breather hose, were actually what the manual calls 'air cleaner drain hose"
 
Any update on your oil leak from Honda? Curious tinkers want to know!
 
So last week they looked at it, couldn’t find any signs of burning oil, they sealed it up and told me to go ride for a thousand kilometres and come back, they took a look at it again, so it seems it’s not burning oil.

The only rational explanation they and I can come up with is if you drop it, oil ends up in places it shouldn’t, that in conjunction with a leak, make it look as if you’re low on oil.

On the one hand I’m relieved, on the other I’m annoyed that Honda didn’t put a freakin sight glass on it.
 
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