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Dark oil

rvdave

Well Known Member
Have approx 500 hrs on a IO360 A1A. Been noticing it doesn't take long for the oil to turn dark. Did compression test recently and all around 79/80. Assuming blow-by, listened with a tube in dipstick hole and can hear a little hiss while testing each cylinder. This was done before a flight but after rotating propeller. Not sure if a little hiss is expected but assuming combustion gases getting to crankcase. Is it advisable to rehone and new rings? Will I need oversized rings if rehoning these steel barrels? Btw not using excessive oil but about a quart every 6-8 hrs but seems to have always been that way.

Dave Ford
RV6
 
Having the cylinders re-honed is a good idea. If done right only a few tenths will be taken out of the bore, and so your rings should be standard if the bore was standard. But if the cylinders have wear steps in them at the top and bottom you might have to go oversize on both pistons and rings in order to take enough material out to get rid of the wear steps.
 
Leave it alone! Wait until the oil consumption is much higher or the compressions start getting low.
 
Have approx 500 hrs on a IO360 A1A. Been noticing it doesn't take long for the oil to turn dark. Did compression test recently and all around 79/80. Assuming blow-by, listened with a tube in dipstick hole and can hear a little hiss while testing each cylinder. This was done before a flight but after rotating propeller. Not sure if a little hiss is expected but assuming combustion gases getting to crankcase. Is it advisable to rehone and new rings? Will I need oversized rings if rehoning these steel barrels? Btw not using excessive oil but about a quart every 6-8 hrs but seems to have always been that way.

Dave Ford
RV6

What is your oil consumption? Does your engine produce the power you expect? Does your exhaust look normal?
 
I agree!

Having the cylinders re-honed is a good idea. If done right only a few tenths will be taken out of the bore, and so your rings should be standard if the bore was standard. But if the cylinders have wear steps in them at the top and bottom you might have to go oversize on both pistons and rings in order to take enough material out to get rid of the wear steps.

I agree completely that this engine is shot. Dark oil is a sure sign that you need to replace this engine with a new one and send this horrible, no good engine to me so I can dispose of it properly:D
 
Dark oil, like deep amber, gray going to black or kinda like black oil?

Having said that. The brand and type of oil will make a difference in its deterioration rate.

Having piston oil jets, advanced timing, and/or consistently high oil temperatures will cause more rapid oxidation of the oil making it blacker and darker quicker. Synthetic oils are not immune, they just take longer and higher temperatures for this to occur.

Keep in mind that oxidation is just one measurement of oil degradation.

If nothing has changed in the engine, CHT, oil temps, fuel burn etc, then you can just reduce your oil change period until a major engine indicator triggers a money-burn.
 
dark oil

Don't do anything until you have unacceptable oil consumption. I have noticed in the last year that my oil was running darker. Considering the other alleged problems I was having with my air/breather set up getting all coked up (see details in other thread), I was convinced I had something wrong with my engine. After chasing everything down, and getting lots of opinions, I quit doing anything. And guess what, nothing has changed. Oil consumption is normal, the engine runs fine.
Blackstone labs tells me they are getting a lot of complaints about dark oil. I don't know if something has changed, perhaps in the oil formulation, or even perhaps in the 100LL blue dye. My oil analysis says I have a little more than the normal fuel in the oil. Perhaps I am running too rich ( I do not fly LOP).

I hope we can keep this thread going and see how many others start coming up with dark oil. :)
Vic
 
How long?

The oil is suppose to turn dark, but after maybe 15-20 hrs. If it's turning dark in like 5 hours, that may be an indicator of bad things, but I would think with that you would also start seeing higher oil temps as the combustion gasses are getting by the rings.
How long does it take to turn, and how dark is it?
Tim
 
Good info

I would watch all of these before spending any time or money taking apart your engine because of something like dark oil:
https://www.savvymx.com/index.php/resources/webinar

I would especially recommend "How Healthy Is Your Engine". Very educational I think. I find Mike Busch's opinion to quite often make more logical sense than the standard things you hear around the aviation world.

Good luck,
 
There is nothing different re temps, power, consumption, or the way it runs. Has always been a smooth strong runnng engine. It has been getting dark for a while within a few hours of oil change. I just really never thought much of it until a post I saw regarding oil color shouldn't be dark nearly as soon. Local a&P opines that when the oil turns dark it's time to change it, it has lost its best lube qualities, which in my case would be changing it every few hours.
Other more modern engines I run---car, boat, -- the oil will remain light colored for many hours or miles-- much more than I'm seeing here.
This engine was rebuilt for my rv6 but just don't recall if it was like this from day one. Have always done oil changes at winter/summer season changes here in northern Mi.
Just wondering about the hissing I hear during compression check in crankcase--is some of that normal when listening via tube in oil fill? Probably not a true test for what happens in a cylinder during combustion but isn't that what would make oil turn dark prematurely is combustion gases getting by the piston instead of exhaust valve? That's why I'm thinking of rehoning and new rings.

Dave Ford
RV6
 
Why wait until things get so bad that you NEED to do something. By that time, doing something will cost more. Here's why. Bad ring sealing will cause scoring of the cylinder walls. and it takes removal of a lot more material to get rid of the scoring and wear step. When you do them by hand like me trust me you see first hand how much longer it takes to get the bore clean and back into round. Then you might have to have oversize rings and pistons, additional expense. For a four cylinder engine thats roughly $500 more.

Its easier to nip this sort of thing in the bud, before you have carbon buildup everywhere and the cylinders get glazed.

I have read Mike Busch's articles and I've come to the conclusion he's never touched the inside of an engine and is a bit of a blowhard. Last month's article was definitely "out there". I digress.

My mower has about 2000 hours, 20 years old...haven't changed the oil in 10 years. Its still nice and golden colored. THAT's the sort of thing you don't touch.

A diesel motor, on the other hand, will turn oil black within minutes of running and that doesn't mean that they have to be re-honed and re-ringed.
 
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Check the color of your oil when it is hot. I noticed that aeroshell 15-50 tends to get dark when checked cold. Much lighter when hot. No idea why. Aeroshell 100 does not seem to get as dark.
George
 
Why wait until things get so bad that you NEED to do something.

I'm curious what symptoms indicate that there may be any sort of problem here. The oil gets dark is a symptom? Isn't that what AD oil is supposed to do?

Doesn't a compression check of 79/80 mean there's not a ring sealing problem? Of course there's going to be some leakage on any compression test. 79/80 is excellent. Even if the measurement were 80/80, you're still likely dumping at least a small amount of air continually into the cylinder, just less than is going through the orifice of the compression tester. That air is going somewhere (likely through the ring gaps and into the crankcase), which is where the OP hears a faint hiss. What part of that indicates a problem that needs to be rectified?

I'm not the smartest engine guy, but that all sure sounds pretty normal to me.
 
I was the maintenance officer for a 172 flying club airplane once. I noticed that the oil suddenly started getting very dark in only a few hours. There we no other symptoms of a problem - compressions good, no metal in filter, and power seemed ok.

It was an O-320-H2AD and had over 2600 hrs on it (it was flown a lot, maybe 400 hrs per year). We decided it was time and opened her up. The dark oil was due to the fact that 2 lobes on the camshaft had worn down aboutt 1/4 inch. Once the hard outer layer is worn through, the cam metal is softer iron and turns the oil black quickly.
 
I'm curious what symptoms indicate that there may be any sort of problem here. The oil gets dark is a symptom? Isn't that what AD oil is supposed to do?

Doesn't a compression check of 79/80 mean there's not a ring sealing problem? Of course there's going to be some leakage on any compression test. 79/80 is excellent. Even if the measurement were 80/80, you're still likely dumping at least a small amount of air continually into the cylinder, just less than is going through the orifice of the compression tester.

I'm not the smartest engine guy, but that all sure sounds pretty normal to me.

You can have a ring that is stuck with carbon and sealing perfectly. You can have good compression tests however when running it can be a different story, with bad blowby and lots of oil usage. I've had rings with decent compression tests and have had sections of the rings fall to the floor while the cylinder was pulled. If the oil is getting dark within 2-3 hours and oil usage is up somethings not right. It may not be a bad problem now but its a lot easier to deal with now.

Also think about where compression tests are done...at TDC. If one ring seals perfectly but the other two don't one doesn't know any different. If they were done at mid-stroke or lower, might get totally different results.
 
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Makes a lot of sense Bob, I think I'm gonna prepare to do this after Osh so I'm not rushing through things.

Dave Ford
RV6
 
Thanks, Bob. I like learning new stuff from you engine gurus.

So if the OP pulls the jugs and the rings are all in one piece and can be rotated by hand on the piston, does that mean all is ok?

That also brings up another question: why DON'T we do compression tests both at TDC and BDC?

Finally, if oil usage is lower (15-20 hours or more per quart), what would be a "normal" time frame for oil to go dark after oil change?
 
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Thanks, Bob. I like learning new stuff from you engine gurus.

So if the OP pulls the jugs and the rings are all in one piece and can be rotated by hand on the piston, does that mean all is ok?

That also brings up another question: why DON'T we do compression tests both at TDC and BDC?

Finally, if oil usage is lower (15-20 hours or more per quart), what would be a "normal" time frame for oil to go dark after oil change?

1. No. The condition of the cylinder wall and the ring face is what matters in addition to the ring gaps.

2. Hard to hold, and more of a risk for injury.

3. If it turns black right away in say 3 hours thats one thing. A gradual darkening over 15 hours is another.

If I shined a light thru an open spark plug hole and saw no crosshatch, and the oil is turning dark quickly, I would re-hone and re-ring. Compressions could be great. Along the same vein, this is why chrome cylinders use oil and oil darkens quickly. The cylinder walls don't retain enough oil to allow the rings to be effective.
 
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