Does oil quantity/capacity affect UOA?

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Jul 6, 2009
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I’m curious, does the amount of oil in the engine sump/pan affect UOA readings? Example if your engine took 5 quarts, but it was a quart low(had only 4 quarts) when you pulled the plug to change the oil and grab a sample?
 
I've wondered this myself. My wife's 3.7L V6 and my 5.3L V8 both take 6 quarts. The same capacity with 2 more cylinders, 2 more sets of rings, 2 more pistons, 2 more rod bearings, etc... I would think if each cylinder is wearing equally, a lower capacity would show a higher wear number in UOA. I could be wrong about that though.
 
I've wondered this myself. My wife's 3.7L V6 and my 5.3L V8 both take 6 quarts. The same capacity with 2 more cylinders, 2 more sets of rings, 2 more pistons, 2 more rod bearings, etc... I would think if each cylinder is wearing equally, a lower capacity would show a higher wear number in UOA. I could be wrong about that though.

I didn’t even think of a scenario like that.

I had recently picked up a used car that was low by a whopping 3 quarts, it’s suppose to take 5. I had to add 3 quarts of “make up/top up” oil. I am debating on running a UOA, which had me thinking, what if I had sampled it without topping the oil off?
 
There must be so many more variables than just sump capacity. Simplifying here. Making assumptions as I'm no engineer but taking 2 of the same engines with the same valve-trains one has a 5 quart sump another has 6 quart sump and oil is run to 7500 miles under the same conditions. Afraid to say it but plain o'l common sense would say all variables equal the lower sump capacity should show the same or increased wear metals. Right?
 
I had recently picked up a used car that was low by a whopping 3 quarts, it’s suppose to take 5. I had to add 3 quarts of “make up/top up” oil. I am debating on running a UOA, which had me thinking, what if I had sampled it without topping the oil off?

I would assume the remaining oil would be deteriorating at a faster rate (less TBN, etc..) than if it was at full capacity.
 
Oil capacity can be a factor to provide a longer OCI when designing an engine.
If your making Koolaid, adding more water dilutes it while adding more Koolaid makes it stronger.
So PPM would be 1/2 the amount if you mixed in 5 quarts of fresh oil in a bucket full of 5 quarts of used oil assuming.
 
It would depend. If its been low since the oil change then the oil contamination is concentrated into 4 Qts rather than 5 Qts. If the oil leaked or was burned some of the contaminants went with it. If you just added a QT of makeup oil now the oil is contaminated 20% less for any given sample.
 
Assuming the oil is doing its job, we can assume the engine is going to wear at a constant rate, right? Therefore, putting an amount of wear metals into 4 quarts of oil will show higher concentration numbers in a UOA than putting the same amount of wear metals into 6 quarts oil. This is simple.

What happens when a 5 quart engine is 2 quarts low at oil change time? Well, those same wear metals are being put into 5 quarts in the beginning. As you drive the car, oil is lost (burned/leaked/whatever) and some of those wear metals are lost along with the oil. Also, as the oil level drops, wear metals are being put into a decreasing quantity of oil. In the end, the wear metals are being put into only 3 quarts of oil. The result is a higher concentration of wear metals. How much higher? Certainly should not be double, as we lost some during the OCI. You need to do some calculus math to figure out how much.

Assuming the oil is doing its job and we can assume the engine is going to wear at a constant rate, the increased concentration of wear metals in the 3 quarts would be expected. If you add make-up oil during the OCI, then simple 'back of the napkin' calculations would show that over the OCI some of the wear metals are lost along with the lost oil and new, clean make-up oil is added that has no wear metal. You should see a reduced concentration of wear metals in a UOA if you added make-up oil and ended with a full 5 quarts of oil when you take the UOA sample at your OCI.

If the oil is breaking down, wear metals will certainly increase, and that is another story.
 
The easy answer to this question is absolutely yes.
However the difference in wear metals at 4 vs 5 qts would not be significant.
IE: if you have 10 ppm Fe at 4 qts that might only be 8 ppm in 5 qts. And those 2 numbers are not significantly different.

If it were more drastic conditions or you were really pushing the OCI then it might be more significant.
 
My 1985 motorcycle took 1.5 qts of oil with a OCI of 1,500 miles.
My 2012 and 2015 motorcycles take 4 qts and the OCI is 8,000 miles.
My 2019 4Runner takes 6.6 qts with a 10,000 mile OCI.
Today's oil is better quality and vehicles use more of it.

The trend is to make longer maintenance intervals so consumers can drive care free. Oil is inexpensive. Why not make a larger sump and extend the OCI? Easier ownership experience.
 
For a given sump, it is the trend that is significant.
I'm not exactly sure what you're saying. But if I knew I was a qt low and ppm were a little higher then I understand the reason for it and the trend is less important than the explanation.
 
I’m curious, does the amount of oil in the engine sump/pan affect UOA readings? Example if your engine took 5 quarts, but it was a quart low(had only 4 quarts) when you pulled the plug to change the oil and grab a sample?

The correct answer is yes, no and maybe depending on the following. This question and scenario often come up in my ML-1,2 and LE trainings and its important in understanding the validity of a OA sample. Here's how this works.

A sump is a bowl and it collects "stuff". These particles ( the small ones are usually below 5 micron and often barely gram positive and some even almost colloidal)

There's no uniform mixer/agitator in an engine so there is no uniform distribution.

This it becomes the quality of the sample- is it truly representative of actual real time wear or am I sampling from a backwater (collection pond) or a rapid (excessively clean).

There is an art and science to installing sample ports in a machine then timing the sample to get a true working sample.

Samples drawn from sump drains and stagnant settled areas are notoriously skewed ( both ways- heavy and light)

Then there's the date stamp ( figuratively)- I got a particle count- is that last months particle? Is it new?

Another weakness when OA alone is used to attempt to monitor and measure wear.

Then come make ups- We document these in industrial lube programs because of the volume ( cumulative over time) combined with displacement and even making sure the make up oil is correct- this can irreparably skew additive data.

So the relative sump volume does have an impact but the measure of that impact depends on a lot of other factors.
 
I have tubes cut to the length of the dipsticks + 1" for each of my engines so the sample is drawn from ~1" below the bottom tip of the dipstick in the oil sump/pan. I take the sample after driving it where the oil is fully warmed up for at least 5 minutes, and draw the sample within 2 minutes of shutting it off. The exception is the track car where that just isn't possible. After the sample, I lightly soak the tube in methanol, blow it out with compressed air, and then hang it to dry. I then blow it out again before the next use.

I draw the sample out with a 100ml syringe which ends up being the exact amount for the fill line on the Wix bottles. I do a similar routine with cleaning it after each use.

I do this regardless if it's a check for that mileage or if I'm about to change it. I'm considering the next oil change to take a sample that way and then catch a sample mid-stream when I change it, send both in, and see how similar (or different) they are.
 
I have tubes cut to the length of the dipsticks + 1" for each of my engines so the sample is drawn from ~1" below the bottom tip of the dipstick in the oil sump/pan. I take the sample after driving it where the oil is fully warmed up for at least 5 minutes, and draw the sample within 2 minutes of shutting it off. The exception is the track car where that just isn't possible. After the sample, I lightly soak the tube in methanol, blow it out with compressed air, and then hang it to dry. I then blow it out again before the next use.

I draw the sample out with a 100ml syringe which ends up being the exact amount for the fill line on the Wix bottles. I do a similar routine with cleaning it after each use.

Take note- This is as good as a well disciplined sampling protocol as exists in industry. ( better than many in the business)

If you ever want to try being a lube tech, let me know.

Now, the question shifts from the sampling process to whether the location selected actually has oil in it that represents current conditions. Its hard enough on a static machine- a car is substantially different and probably more difficult but this should be considered strongly.

That's actually the most important (and usually least considered) part of the OA everywhere and more often than not requires an engineering review to get a proper sample.

Its one thing to get a bottle of goo that you send off with a $50 to get a pretty report with all kinds of stuff on it.

Its another one to get one where that stuff is actually relevant and can be used for intelligent decisions.

I point this out again because if the sample proper isn't directly relevant to actual wear conditions- the report is worthless and any decisions made from it are just crap shoots.
 
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