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High Crankcase pressure

wildduk

Member
I had a previous post about crank seal but wanted to start a new one on possibly related subject....
O360, 300smoh. Crank seal won’t hold. High oil burn, 5hr/qt.
Mechanic said compressions were good.
I’ve read on other forums where a broke or stuck oil ring will cause high crankcase pressure. But is it possible to have good compression and still have a stuck or broke oil ring. I’m thinking it’s possibly causing the crank seal issues as well as the increased oil use.
The crankcase-pressure has not yet been tested.

Thx
 
What’s the condition of your crankcase breather line? It seems like if this wasn’t pinched, or restricted in some way, you wouldn’t have crankcase pressure buildup. In that case, if you had a broken oil ring on one or more cylinders, you should see oil on your belly. Maybe I don’t understand how a crankcase breaths, but it makes sense to me.
 
OIL

If the 0 360 is parallel valve try keeping the oil level slightly below 7 quarts. Let the oil level go down to 6 1/4 and then add 1/2 quart. Minimum for the parallel valve is slightly over 2 quarts.
 
compression and oil burn are not necessarily related. Glazing is a good example. COmpressions can be great, but burn large amounts of oil.

I recently had a cracked compression ring (50/80) and had NO observable pressure increase in the crankcase and certainly didn't blow out my main seal. The breather can pass s LOT of air and blowby shouldn't increase C/C pressure by any meaningfull amount.

If you are blowing out main seals, you either have a restriction you haven't yet observed or the seal is under sized (some engines need oversized seals) or improperly installed.

Larry
 
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I’ve read on other forums where a broke or stuck oil ring will cause high crankcase pressure. But is it possible to have good compression and still have a stuck or broke oil ring. I’m thinking it’s possibly causing the crank seal issues as well as the increased oil use.
The crankcase-pressure has not yet been tested.

Thx

Having served as a design engineer in an engine group where my engine had field issues with both blow-by and high oil consumption . . .

First: While a worn rings with large end gaps or scoring typically accompany worn oil rings (wide faces/low tension) that no longer scrape the oil it is easy to conclude they result from the same "failure" but they don't. The cause of blow-by and oil consumption come from two completely different causes.

The oil control ring really has to do it's function, that is scraping down the cylinder wall and not sliding over an oil film. That is completely separate from blow-by. A fact.

If too much oil is on the cylinder walls, the oil ring can be flooded and exceed its ability to drain resulting in increased oil consumption. Like running excessive oil level in the crankcase. Be sure that is not a factor for you.

Blow-by is leakage past the rings and through the gaps. You could leak down test nicely with one good top ring, but if the second ring is bad you can still have higher blow-by. Or vice-versa.;) A cylinder wall inspection will give clues to this failure mode.

And . . A good visual inspection of the cylinder walls, and a reading of the spark plugs etc would help identify if just one jug is at fault.

Fundamentally, an inspection and physical test, of the crankcase vent system is certainly in order early in your diagnostic journey. Blow though it.

Good luck with your investigation.
 
Two Suggestions:

One, drop your oil fill level to 6 Qt, It will run fine with 5.
Two, You can test the crankcase pressure by temporarily plumbing a vinyl line from the oil dipstick/ filler opening to a spare airspeed indicator in the cockpit.
the indication in flight should be at of below 50 Kt.
If it indicates excess crankcase pressure, try adjusting the location of the vent tube termination.
 
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