Running an engine with blow-by?

W

Whattodo

Guest
Our airplane recently developed blow by. Engine is an O360A4M 180hp. The oil turns dark within a few hours and burns around 8hrs a qt. I know we’re leaking past the rings, as I can hear it hissing from the oil filler tube when I pull the prop through. The compressions at last annual were all in the 70’s (low to mid). The plugs are rather clean and not oily. It runs well knock on wood. Is it okay for us to fly it for awhile? We fly it often, like multiple times a week.
 
So what dipstick level are you keeping the oil at? Lycomings don't like to be filled to the max. They will blow a quart out in short order.

Oil will turn dark quickly. It's normal.
 
Dan Thomas said:
So what dipstick level are you keeping the oil at? Lycomings don't like to be filled to the max. They will blow a quart out in short order.

Oil will turn dark quickly. It's normal.
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I fill it to 7 when I do an oil change and it usually takes around 8 hrs to burn through it until it gets down to 6. And then I keep it around 6 til I change the oil again at 25hrs.

Is it normal? I always thought oil was supposed to stay relatively clean looking til it got beyond 30-35hrs? Mine begins to look pretty dark after around 10 hrs. And it’s pretty opaque by 25.
 
Whattodo said:
The oil turns dark within a few hours and burns around 8hrs a qt.
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In my experience it depends on the engine. Knew some that stayed "clean" for a period and then knew some engines where it seemed the oil turned dark before you finished pouring it in especially on radials.
Per the book, you can burn between .39qt to .80qt per hour depending on cruise settings which current burn rate of .125qt/hr is good.
Is this the only issue of concern whether to fly it or not given you have good compressions and plug conditions?
 
Bell206 said:
In my experience it depends on the engine. Knew some that stayed "clean" for a period and then knew some engines where it seemed the oil turned dark before you finished pouring it in especially on radials.
Per the book, you can burn between .39qt to .80qt per hour depending on cruise settings which current burn rate of .125qt/hr is good.
Is this the only issue of concern whether to fly it or not given you have good compressions and plug conditions?
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Basically yes, is the fact that it has blow by and the oil turns opaque rather fast impact us flying it? At what point should we not?
 
Whattodo said:
Basically yes, is the fact that it has blow by and the oil turns opaque rather fast impact us flying it? At what point should we not?
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All engines have blowby. The real question is, how bad is it or what else is wrong with the engine. From what is being described here, I’d guess there isn’t anything wrong with it but nobody on the site has laid hands on it to give you an absolute answer.

Find a good engine mechanic and trust their diagnosis.
 
The following formula is used to calculate the maximum allowable oil consumption limits for all
Lycoming aircraft engines.
0.006 x BHP x 4 ÷ 7.4 = Qt./Hr.
From here:

On your 180hp motor that = .58 qt/hr


For compression:
If the pressure reading is below 60 psi or if the wear rate increases rapidly, as indicated by appreciable decrease in cylinder pressure, removal and overhaul of the cylinders should be considered.

From here:
 
Whattodo said:
Is it normal? I always thought oil was supposed to stay relatively clean looking til it got beyond 30-35hrs? Mine begins to look pretty dark after around 10 hrs. And it’s pretty opaque by 25.
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There is always some dirty oil still in the engine after you drain it. It will instantly color the new stuff somewhat. New, clean oil also scrubs that engine, so it's picking up stuff, too. And heat discolors oil quite quickly as well.

These are not car engines. They have loose tolerances so that they don't seize when hot. They operate at higher temps than liquid-cooled engines. Their oil gets dirty and dark real quick.
 
FORANE said:
For compression:
If the pressure reading is below 60 psi or if the wear rate increases rapidly, as indicated by appreciable decrease in cylinder pressure, removal and overhaul of the cylinders should be considered.

From here:
https://www.lycoming.com/sites/default/files/attachments/Cylinder%20Compression.pdf
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Well, yes Lycoming has a Service Instruction that says that, but it does say "consider," and these days more data can be gathered to use in that consideration. That SI was written in 1998, when borescopes cost a fortune, didn't have terrific imagery, and hardly anyone had one. These days a really good scope can be had for two or three hundred bucks.

A compression reading below 60 should probably be re-checked a time or two after flying for a while. If it stays low, then borescoping is in order to make a close inspection of the valves. Let the borescope tell you if and when it's time to pull the cylinder, not just a somewhat inaccurate and unreliable compression reading.

In this case, though, the OP's jugs all have compressions in the 70s, so I certainly wouldn't be thinking of pulling a cylinder. Furthermore, his oil consumption is completely normal for that engine.
 
Agreed on all. I have a borescope and use it regularly. Just throwing out some specific data points from the manufacturer. Those data points the OP may notice are far from his current observations.
 
Whattodo said:
Our airplane recently developed blow by. Engine is an O360A4M 180hp. The oil turns dark within a few hours and burns around 8hrs a qt. I know we’re leaking past the rings, as I can hear it hissing from the oil filler tube when I pull the prop through. The compressions at last annual were all in the 70’s (low to mid). The plugs are rather clean and not oily. It runs well knock on wood. Is it okay for us to fly it for awhile? We fly it often, like multiple times a week.
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You're describing a perfectly normal, healthy engine. Fly it.
 
same engine in my pa28 - oil turns dark after 2 flights, consumes 1QT every 4 hours and has been doing this for last several years (all compression mid 70s) - I thought it's high, mechanic says 'normal'. I just fly and remember to add oil after each long x-country..
 
Whattodo said:
At what point should we not?
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Can't answer that for you. But as mentioned above, what you have described is not out of the ordinary. However, if you have doubt then have a mechanic check things and take it from there.
Out of curiosity, did you develop this inquiry solely based on your personal observations or did some 3rd party convince you the oil discoloration and "blow by" was a potential problem?
 
Alien Mind said:
same engine in my pa28 - oil turns dark after 2 flights, consumes 1QT every 4 hours and has been doing this for last several years (all compression mid 70s) - I thought it's high, mechanic says 'normal'. I just fly and remember to add oil after each long x-country..
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No kidding! Sounds exactly like mine. How much do you fly a year? Has it always burned that much or did consumption increase as the engine aged/wear?
 
Which oil are you using?

You said “developed blow-by”. Suddenly? What time period?
 
Stewartb said:
Which oil are you using?

You said “developed blow-by”. Suddenly? What time period?
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20w-50. I noticed it after awhile, say several months? After shutdown, the exhaust fumes have a rather burnt smell after shutdown.
 
The thing you’d notice with increased blowby is oil on the floor under the breather and oil on the belly. I’d guess you’re leaking oil through valve guides.
 
Alien Mind said:
same engine in my pa28 - oil turns dark after 2 flights, consumes 1QT every 4 hours and has been doing this for last several years (all compression mid 70s) - I thought it's high, mechanic says 'normal'. I just fly and remember to add oil after each long x-country..
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One quart in 4 hours is not "normal." That's 12 quarts in 50 hours. Are you filling it to the maximum?

In 1999 Lycoming changed their valve guide material from bronze to a high-chromium-content bronze. That stuff doesn't wear like the old stuff did. But there are still lots of older engines out there with the old guides, and worn guides lead to higher oil consumption.

A quart in four hours should be fouling the sparkplugs real fast. If that's not happening, I'd suspect oil escaping out the breather due to overfilling. "High 70s" compressions still means blowby, and those gases are going to carry oil mist out of the breather. More oil mist forms in the case if the oil level is high.
 
Stewartb said:
The thing you’d notice with increased blowby is oil on the floor under the breather and oil on the belly. I’d guess you’re leaking oil through valve guides.
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There is. There’s always a half dollar ish sized drop below the breather after a flight.
 
Whattodo said:
There is. There’s always a half dollar ish sized drop below the breather after a flight.
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A small drop of oil on the floor means nothing in my book. It is more indicative of variation in crankcase breather configurations, from what I’ve seen. 100% healthy engines can and will make their presence known the same way.

You’ve provided no evidence that there is anything wrong with this engine so far. Talk to an experienced engine guy if you continue to be concerned with it.
 
mondtster said:
A small drop of oil on the floor means nothing in my book. It is more indicative of variation in crankcase breather configurations, from what I’ve seen. 100% healthy engines can and will make their presence known the same way.

You’ve provided no evidence that there is anything wrong with this engine so far. Talk to an experienced engine guy if you continue to be concerned with it.
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Thanks, does the burnt smelling exhaust odor that I can smell shortly after shutdown not indicate anything to be concerned with?
 
Whattodo said:
Thanks, does the burnt smelling exhaust odor that I can smell shortly after shutdown not indicate anything to be concerned with?
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It could be an oil leak rather than “blowby” take it to a shop if you’re concerned.
 
Dan Thomas said:
One quart in 4 hours is not "normal." That's 12 quarts in 50 hours. Are you filling it to the maximum?

In 1999 Lycoming changed their valve guide material from bronze to a high-chromium-content bronze. That stuff doesn't wear like the old stuff did. But there are still lots of older engines out there with the old guides, and worn guides lead to higher oil consumption.

A quart in four hours should be fouling the sparkplugs real fast. If that's not happening, I'd suspect oil escaping out the breather due to overfilling. "High 70s" compressions still means blowby, and those gases are going to carry oil mist out of the breather. More oil mist forms in the case if the oil level is high.
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I fill it just to 6 qts, as anything above that just blows out immediately. spark plugs not fouled. based on your comments, perhaps I should consider an old fashioned 'top overhaul' although I know these days, the benefits of that are disputed by gurus such as Mike Busch..
 
Whattodo said:
No kidding! Sounds exactly like mine. How much do you fly a year? Has it always burned that much or did consumption increase as the engine aged/wear?
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The resident A&Ps here gave ya some good next steps to follow if you're still concerned...

But I'll just add that I fly my PA28R multiple times per week and I have the same thing (it's a Lycoming IO-360-C1C6).
It will burn approx 1 - 1.25 qts every 10 hours based on where I'm at in terms of hour since oil change. It burns a bit more from 30-50h post oil change (and I always change <= 50 hrs).
Like yours my oil is dark within 15 hours. When I say dark, I mean dark (not like "ok, it's starting to darken a little").
In my oil analysis program through blackstone we've never noticed any out of the ordinary contaminants that spell danger, nor has it manifested problems in other ways. This has been the case for the last 3 years and 530 hours in the plane.
I'll join the chorus of folks saying you're probably OK :D
 
Whattodo said:
No kidding! Sounds exactly like mine. How much do you fly a year? Has it always burned that much or did consumption increase as the engine aged/wear?
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About a 100 hours per year in the last 5 years (with a low of 70 last year) - fairly sure it was 1QT every 5 hours when I bought it in 2019 so not a huge change.
 
Alien Mind said:
About a 100 hours per year in the last 5 years (with a low of 70 last year) - fairly sure it was 1QT every 5 hours when I bought it in 2019 so not a huge change.
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Bad to begin with. The 172s I had, with the O-320s, might have used three or four quarts in 50 hours. We tracked it.
 
FORANE said:
The following formula is used to calculate the maximum allowable oil consumption limits for all
Lycoming aircraft engines.
0.006 x BHP x 4 ÷ 7.4 = Qt./Hr.
From here:

On your 180hp motor that = .58 qt/hr


For compression:
If the pressure reading is below 60 psi or if the wear rate increases rapidly, as indicated by appreciable decrease in cylinder pressure, removal and overhaul of the cylinders should be considered.

From here:
Click to expand...

Not to nit pick, but it was pointed out to me several years ago that you have to factor in the power setting. Since most only achieve ~ 75% power the actual figure is closer to 0.44 qt/hr...
 
idahoflier said:
Not to nit pick, but it was pointed out to me several years ago that you have to factor in the power setting. Since most only achieve ~ 75% power the actual figure is closer to 0.44 qt/hr...
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So Lycoming thinks .44 quarts per hour is OK. Maybe it it is, but that gets expensive, fouls plugs, carbons things up.

Continental thinks compressions in the low 40s are OK.

Champion thinks sparkplug resistors having way over 5000 ohm resistances are OK. Infinity, even. They even doubled down on their spring-loaded resistor being just fine, until we here made a lot of noise about early plug failures and mag drops and high resistances, and how Tempest plugs were so much better, and they changed their design.

My old Auster's Gipsy Major engine would use a quart of Aeroshell 100 per hour. When it went to Texaco 100 it dropped to half a quart per hour. Such numbers, in 1940s inverted inline engines that are at or over their TBOs, is to be expected. When I took it apart, there were massive piston-to-cylinder clearances and the rings were shot. Even then, most of the oil was going past the tappets and down the pushrod tubes and filling the rocker boxes until they overflowed out their overflow tubes. Didn't have to worry about the 25-hour rocker box refilling.

But half a quart per hour in a modern Lycoming?? Not when my flight school airplanes got 12 or 15 hours per quart.
 
My 55 year old O320 with original cylinders would go about 7 hrs a quart but it did seem to hover around 5.5 quarts a lot better than 6 to 8.
 
In my best crocodile Dundee accent……


That’s not an engine with blow by. THIS is an engine with blow by.
IMG_2165.jpeg
 
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