IS OIL ANALYSIS A GOOD DETECTIVE?

chemgeek said:
Having done my own oil analysis (I have access to both ICP-OES and now TXRF instrumentation that is ideal for measuring trace metals in liquid samples) periodically over the years I would say that it can be informative but of limited usefulness. It's not a panacea for verifying general engine health. Bear in mind that spectral analysis will only tell you about microparticle wear (we are talking about suspended metal particles that are micron-sized). It will not necessarily tell you if you are making bigger bits of metal that could be more urgent. Over the years my oil analysis has often told me things I have found out anyway at annual time or during routine maintenance: my air filter needed changing (higher than usual silicon), I haven't flown in a while (higher than usual iron, probably from minor cylinder corrosion), or I'm getting blowby into the crankcase (high lead readings). While my students and I found the various oil analyses interesting, ultimately, the information was rarely specific enough to make a specific maintenance decision about. When my last engine was reaching end of life, with a worn cam and lifters that were starting to spall, the oil analysis didn't say anything unusual, and the engine was running great other than excessive blowby and an oil-fouled plug in one cylinder which got it pulled for repair. Once the cylinder was pulled, the cam and lifter wear became apparent.

Spectral oil analysis is kind of like high tech medical imaging: you can find all sorts of interesting things happening that are likely normal and harmless, but if not used in concert with other information, can cause undue and unnecessary anxiety.
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so there is oil passages within the crank shaft itself? Certainly makes sense but I had no clue...
 
Huckster79 said:
so there is oil passages within the crank shaft itself? Certainly makes sense but I had no clue...
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Yulp! The oil is actually there to keep the metals from touching each other by having a pressurized film of oil across the faces of the bearings. This also aids in removing heat from the area.

There will be some leakage from these areas. As they wear the pressure will drop. If a single bearing were to go bad you may not see enough of a drop to catch it but as engines break-in then eventually wear out, the clearances or "gaps" become larger so resistance to flow is less resulting in less pressure.
 
This is a Continental, but the principles are the same as a Lycoming. The red lines represent oil flow.

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This is an automotive V-8. The light blue lines on the crankshaft represent the oil flow from the main bearings to the rod journals.

upload_2021-2-23_19-9-22.png
 
Tom Martino said:
Recently my Ercoupe's C-85 lost oil prime which resulted in no oil pressure on start-up. Unfortunately, it ran for around 15-minutes - including a run-up, before being noticed. Long story, it was a "friend" warming up plane for oil change and did not notice.

I know, I know, the engine is ruined, right? Except, I want to add some details ... the operating temperature never got beyond the low end of the green arc. Then, after priming the oil pump, I re-started it and it ran and sounded great (did not fly yet). I then sent the oil in for an analysis, which showed iron, aluminum and tin a bit high ... but lab noted it is not unusual for an engine with under 100 hours (mine had 76 at the time).

Since then I've changed the oil and filter to run some more before testing the oil again. I haven't flown it since the "incident".

The engine starts immediately, runs great, no noises, no high temps, no unusual behavior.

Conventional thought says the engine is shot ... but ... is that a 100% certainty?
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While oil analysis can be a good indicator over time of abnormal wear it does not always predict catastrophic failures. I once fly an old 310 using oil analysis over a year with no worrisome signs and still had a crankshaft snap in an engine with 1400 hrs.
 
Wow!
All the “so called” mechanics and technical experts that have such a poor understanding of the dynamics of a small Continental engine.

This topic was beat to death on another forum. Finally, someone familiar with the OIL PRESSURE GAUGE SYSTEM on a small Continental gave a reasonable answer.

the oil in the line to the GAUGE lost its prime. NOT THE ENGINE OIL PUMP.
Because the line bled out leaving AIR in the line, no pressure was indicated by the gauge... Doesn’t mean there wasn’t any oil pressure through the engine...!
Everyone ASSUMES no oil pressure means NO OIL pressure. Just means the GAUGE didn’t detect pressure. (Air in line is MUCH more compressible than oil).

I had a leak behind the panel on my C150 (0200A). The copper oil pressure line had to be replaced. Prime was lost. First time run after repair revealed no pressure! I asked the recently retired Delta mechanic newly hired by the shop if he’d bled the line to restore the prime?
He looked at me with a blank stare. Other older mechanic showed him how.......
Instant oil pressure on start up, after bleeding the air from the line.
Engine most likely OK. FIND ANOTHER SHOP/Mechanic!!!

BTW, un-bled brake lines don’t give much pressure either!!!

The only Ercoup I’ve ever flown would do the same thing If left unflown for a time. New oil lines and gauge corrected the issue.
I ferried a Comanche this week that had the legacy Manifold pressure gauge doing the same thing. When engine warmed up, the crack in the line at the fitting on the engine expanded and sealed... E.I. ENGINE monitor wasn’t affected....
I’ve had an Aztec and two C182’s do the same thing....
50-60yr old metal showing fatigue...
 
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AeroLudite said:
Wow!
All the “so called” mechanics and technical experts that have such a poor understanding of the dynamics of a small Continental engine.

This topic was beat to death on another forum. Finally, someone familiar with the OIL PRESSURE GAUGE SYSTEM on a small Continental gave a reasonable answer.

the oil in the line to the GAUGE lost its prime. NOT THE ENGINE OIL PUMP.
Because the line bled out leaving AIR in the line, no pressure was indicated by the gauge... Doesn’t mean there wasn’t any oil pressure through the engine...!
Everyone ASSUMES no oil pressure means NO OIL pressure. Just means the GAUGE didn’t detect pressure. (Air in line is MUCH less compressible than oil).

I had a leak behind the panel on my C150 (0200A). The copper oil pressure line had to be replaced. Prime was lost. First time run after repair revealed no pressure! I asked the recently retired Delta mechanic newly hired by the shop if he’d bled the line to restore the prime?
He looked at me with a blank stare. Other older mechanic showed him how.......
Instant oil pressure on start up, after bleeding the air from the line.
Engine most likely OK. FIND ANOTHER SHOP/Mechanic!!!

BTW, un-bled brake lines don’t give much pressure either!!!

The only Ercoup I’ve ever flown would do the same thing If left unflown for a time. New oil lines and gauge corrected the issue.
I ferried a Comanche this week that had the legacy Manifold pressure gauge doing the same thing. When engine warmed up, the crack in the line at the fitting on the engine expanded and sealed... E.I. ENGINE monitor wasn’t affected....
I’ve had an Aztec and two C182’s do the same thing....
50-60yr old metal showing fatigue...
Click to expand...

Think you may have meant air is MUCH more compressible than oil?
 
AeroLudite said:
Wow!
All the “so called” mechanics and technical experts that have such a poor understanding of the dynamics of a small Continental engine.

This topic was beat to death on another forum. Finally, someone familiar with the OIL PRESSURE GAUGE SYSTEM on a small Continental gave a reasonable answer.

the oil in the line to the GAUGE lost its prime. NOT THE ENGINE OIL PUMP.
Because the line bled out leaving AIR in the line, no pressure was indicated by the gauge... Doesn’t mean there wasn’t any oil pressure through the engine...!
Everyone ASSUMES no oil pressure means NO OIL pressure. Just means the GAUGE didn’t detect pressure. (Air in line is MUCH more compressible than oil).

I had a leak behind the panel on my C150 (0200A). The copper oil pressure line had to be replaced. Prime was lost. First time run after repair revealed no pressure! I asked the recently retired Delta mechanic newly hired by the shop if he’d bled the line to restore the prime?
He looked at me with a blank stare. Other older mechanic showed him how.......
Instant oil pressure on start up, after bleeding the air from the line.
Engine most likely OK. FIND ANOTHER SHOP/Mechanic!!!

BTW, un-bled brake lines don’t give much pressure either!!!

The only Ercoup I’ve ever flown would do the same thing If left unflown for a time. New oil lines and gauge corrected the issue.
I ferried a Comanche this week that had the legacy Manifold pressure gauge doing the same thing. When engine warmed up, the crack in the line at the fitting on the engine expanded and sealed... E.I. ENGINE monitor wasn’t affected....
I’ve had an Aztec and two C182’s do the same thing....
50-60yr old metal showing fatigue...
Click to expand...
You seem to have never studied the Gas Laws and Pascal's Principle. Pascal says that in a closed system, the pressures at any point in the system will be equal once all fluid movement has stopped. That means that even if there is air in the gauge line, once the engine oil pressure has forced oil into that air-filled line until it can't force any more in there, the pressures of both the oil and air are equal. The gauge will read correctly. The advantage of bleeding the line is that the gauge will rise sooner, especially if the fitting at the engine end is the restrictor type. In cold weather that oil will be thick and slow to move, so we used to blow the oil out of the line and replace it with 5606 hydraulic fluid, which has a much lower pour point and will move handily at low temps.

Once again: air in the line does not prevent the gauge from reading. Such an assertion runs counter to all the laws of physics involved here. It just makes you wait longer for the pressure to show. And no, bleeding the gauge line will not fix the loss of prime in a leaking oil pump in a small Continental. I have had plenty of experience with that.

But I'm just one of those old "so-called 'mechanics'" that used to teach this stuff in college...
 
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AeroLudite said:
Air in line is MUCH more compressible than oil
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AeroLudite said:
Instant oil pressure on start up, after bleeding the air from the line.
Click to expand...
FYI: While liquids do resist compression, your premise on a closed indicator system is not quite accurate. As mentioned above, the gauge will eventually read the pressure of the combined air and oil. For example, where an oil indicator needle oscillates due to system vibrations, one of the "so called mechanic" tweaks is to induce a bit of air into the indicating system lines to dampen out the oscillations internally and provide a more accurate reading.
 
So, no need to bleed brakes, or oil pressure lines?
Hmmmm, Never knew that!!!

ever tried to stop a car after a brake job w/o bleeding brakes?
Yeah, they’ll EVENTUALLY pump up. But not before totaling the car (or airplane).

And, who’s going to run an aircraft engine with no oil pressure long enough to build pressure in the line just to see “if it’ll come up”?

I in fact had a mechanic do that to my Cessna 150 in 2008 during a 100hr inspection. (Forgot to put oil in it!) I heard about it 3rd hand... Cost shop an overhaul of my engine at Don’s Dream Machines...
Yeah, I know a little about Boyle’s laws...
 
AeroLudite said:
Never knew that!!!
Click to expand...
FYI: not everything is an absolute in aviation. Small Continentals can lose prime at the oil pump. And not all brake systems should have all the air bled from the system as in Stearmans--otherwise you go through a lot of props, engines, and vertical fins due to the brakes locking up. So it's relative to the situation at hand.
 
AeroLudite said:
So, no need to bleed brakes, or oil pressure lines?
Hmmmm, Never knew that!!!

ever tried to stop a car after a brake job w/o bleeding brakes?
Yeah, they’ll EVENTUALLY pump up. But not before totaling the car (or airplane).

And, who’s going to run an aircraft engine with no oil pressure long enough to build pressure in the line just to see “if it’ll come up”?

I in fact had a mechanic do that to my Cessna 150 in 2008 during a 100hr inspection. (Forgot to put oil in it!) I heard about it 3rd hand... Cost shop an overhaul of my engine at Don’s Dream Machines...
Yeah, I know a little about Boyle’s laws...
Click to expand...
Air in brake lines is a problem, because the available volume to compress it is fixed. That master cylinder can deliver only so much fluid in one stroke. In the engine, that oil pump keeps pumping oil until the engine runs out of oil. Big, big difference.

If you had air in your brake lines, and had a pressure gauge at the master cylinder and another at one of the calipers, and pushed once and held it, you'd see the same pressures at both places. Pascals' Law. Air is compressible, hydraulic fluid is incompressible, but that makes absolutely no difference in pressures once the fluid motion has ceased.
 
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