How to check oil at a prebuy?

[bcool]

If I'm having a prebuy done on an engine that has recently had an oil change, is there any chance of getting a true representative sample for analysis? I would be suspicious that the owner was trying to hide something if they did a change right before putting the plane up for sale. Maybe not, but is there any way to reasonably check for any issues?

 

[MooneyDriver78]

I’d ask for history of oil sample analysis and engine monitor logs.
How many hours on the engine?

 

[bcool]

I don't have one in mind right now, I'm just getting ready for any eventualities

 

[Let'sgoflying!]

hide something if they did a change right before putting the plane up forMaybe not, but is there any way to reasonably check

engine log books
3 years of changing oil every 45-55 hours but the last one was at 15 hours would sound fishy

 

[Fearless Tower]

I would be suspicious that the owner was trying to hide something if they did a change right before putting the plane up for sale.

Why would you be suspicious?

Most people don’t realistically expect to have the plane sell the first week they list it.

A lot of planes sit in the market for a few months before they find a buyer. And in many cases, the owner is still flying it in the meantime.

When I’ve listed airplanes for sale, the timing of the oil change was not even on my mind.

 

[Fearless Tower]

engine log books
3 years of changing oil every 45-55 hours but the last one was at 15 hours would sound fishy

Even that isn’t damning. If the airplane didn’t get much use in the past year, the owner may have wanted to change the oil based on calendar time vs hours. A lot of planes for sale have reduced flying hours in the most recent year/s for various reasons which may be tied to why the owner is now selling.

 

[mondtster]

To answer simply, there is not a good way to analyze the oil or filter contents on an oil change with only a few hours on it and get much meaningful information. The same problem exists when airplanes only get flown a few hours between annuals.

In these circumstances you do the best you can to determine engine health. I would suggest considering the overall condition of the airplane and it’s past history rather than placing so much weight on an oil sample. There are plenty of other indicators that can be used to determine if the plane is likely good or not, and even the best airplane still will have risk involved with the purchase.

 

[WannFly]

Hours doesn’t mean a whole lot here, last winter I wasn’t flying much so I changed oil at 3 month interval with 10 hrs on it

 

[Fearless Tower]

To answer simply, there is not a good way to analyze the oil or filter contents on an oil change with only a few hours on it and get much meaningful information. The same problem exists when airplanes only get flown a few hours between annuals.

In these circumstances you do the best you can to determine engine health. I would suggest considering the overall condition of the airplane and it’s past history rather than placing so much weight on an oil sample. There are plenty of other indicators that can be used to determine if the plane is likely good or not, and even the best airplane still will have risk involved with the purchase.

This is really the best answer to the OPs question. The condition of the airplane as a whole will tell you more than an oil analysis.

It is rare that an owner takes great care with the airframe and neglects the engine. And likewise, unless it is a fresh overhaul, if the airframe is neglected, the engine probably is too.

When I was shopping for my Beech 18, am expert in the type gave me this piece of advice: look in the gear wells. That will tell you a lot about how well the airplane has been taken care of.

 

[benyflyguy]

I usually go by taste...

 

[Jumpmaster]

Besides, you need a series of oil analyses to make any sense of the trend inside the engine. A one shot sample won’t really tell you much. I really wonder how many owners take an oil sample each time they change the oil and get it analyzed. My suspicion is that it’s a pretty low number. When I purchased my plane, the prior owner had taken 2 samples which revealed exactly nothing. A year later when I had a top overhaul done, my A&P got a very good look at the cam, lifters, etc. and found everything was in great shape. Interesting thing is I had the Lycoming wobble test performed as part of the pre-purchase and all cylinders checked out and all had solid compression. Ironically it was my lowest time cylinder - a new Superior Millennium with only 200 hours on it - that showed zero compression at the next annual and 75lbs at purchase.I say ironically because the prior owner was a Mike Bush fan so the engine had 4 cylinders of various pedigrees including one that was .10 over. If anyone cylinder would have failed, I would have guessed that it would have been the one. Anyway, point is, ya never can tell.

 

[Tom-D]

Besides, you need a series of oil analyses to make any sense of the trend inside the engine.

Unless it has sparkling stuff in it.

 

[NordicDave]

if it has 15 hours on the current oil. Why not fly a couple hours on a demo flight, bring it back to the hanger, and remove the filter cut it open and inspect metal?

 

[Tom-D]

Many older engine run without filters, so they are on a 25 hour cycle, so the oil change is due.
take the sample

 

[Salty]

Historical trend is much more important than a recent single point. If there aren't records of that sort of history, I'm not sure there's much of a point in doing an oil analysis at pre buy.

 

[Kenny Phillips]

engine log books
3 years of changing oil every 45-55 hours but the last one was at 15 hours would sound fishy

That would depend on the time period; we changed ours at an hour usage and after a certain time if those hours didn't happen.

 

[NordicDave]

if it has 15 hours on the current oil. Why not fly a couple hours on a demo flight, bring it back to the hanger, and remove the filter cut it open and inspect metal?

Many older engine run without filters, so they are on a 25 hour cycle, so the oil change is due.
take the sample

Tom, I was assuming the plane had a filter given the 45-55 hour oil changes. In any regard, you’re obviously correct many engines don’t have a filter. If the engine only has an oil screen, he should probably pass on the plane as the oil should likely have been changed at half that interval.

 

[MooneyDriver78]

If I know the plane is going to be idle, I change the oil regardless if it’s not due.


Tom

 

[Tom-D]

Tom, I was assuming the plane had a filter given the 45-55 hour oil changes. In any regard, you’re obviously correct many engines don’t have a filter. If the engine only has an oil screen, he should probably pass on the plane as the oil should likely have been changed at half that interval.

given the engine has a filter, it has enough time to take a sample.