Engine Life (MTBF) and oil/OCI - statistics

[glenncof]

There is a probability of failure related to a particular engine, oil/OCI, and driving style. This is vastly over simplified since every instance is unique. Nevertheless there is a statistical concept used to predict life expectancy called MTBF (mean time between failure) which is helpful to understand the impact of certain vehicle maintenance.

MTBF is a calculation which can be/is done by the manufacture based on all the failure modes observed and analysis of failure mechanisms. Of course two of these variables are engine oil and OCI (oil change interval)

Let’s assume a manufacture sets oil/OCI to achieve MTBF that is cost effective* to say 120K miles. By definition MTBF means 50% fail before and 50% after MTBF. So in this case, half of the engines are expected to last 120K and half fail based on recommended service interval. Nevertheless the average life expectancy is 120K.

There is a probability distribution which is calculated from the failure modes called "Standard Deviation" (aka S.D. or sigma). It is used to predict the probability of failure relative to the average or MTBF.

Statistically +/- one SD from the MTBF covers 67% of failure population. Lets say SD=20K miles, then using above, 67% of engines likely to fail between 100K and 140K miles.

Two SD covers 97% probability. Three SD 99.6%. So in this case there is 3% probability that you'll fail before 80K or exceed 160K. Assuming 'normal' distribution, there is a 1.5% chance fail before 80K and 1.5% chance fail after 160K. You can see how this is useful to predict likely warranty costs in large sample size.

Now put aside what actual MTBF or SD predicted for your engine since you can not control that. These can be calculated by the manufacture and may be set to minimize owner cost* over expected vehicle life. What you can do is change the probability of your failure by changing oil, OCI, or other factors, such as driving habits.

*Cost effectiveness may assume dealer changes that may be more expensive than DIY.
This would lead to longer OCI recommendation. Sure we can service more often but there is a price to pay and is it worth it compared to the future cost of repair.

For myself, I use most vehicles between 140-200K so my target MTBF is much longer. At the same time I want to reduce SD or surprise factor ! This implies an improved service over the manufacture’s recommendation. Two things to look at is improved oil or reduced OCI.

Ideally there is no increase in oil consumption as vehicle ages. Unfortunately it takes awhile to determine this and after damage is done the best you can do is stabilize it at that point.

My current example is a 2005 Ford Duratec 3L engine. Factory spec is MC 5W20 blend with 5K OCI (normal) and 3K OCI (severe duty). Warranty requires 5K OCI minimum.

I used MC blend until 18K miles except one run of dino after factory fill. OCI during this time was 3K. I wanted short OCI until about 20K. At that point I changed to a GrpIII synth (PPqt) and extended the OCI to 5K. I feel this is conservative but is a cost effective way to increase engine life probability.

Each situation is different but the concept is the same; better oil with reduction in OCI done in a cost effective manner can extend life, reduce uncertainty, and reduce overall ownership cost.

 

[B00SS]

Oh, gee thanks, glenncof. Just when I was getting comfortable with an extended drain. Now what do I do?

 

[PT1]

None of those calculations mean anything unless you also calculate things like oil type, ambient air temp,fuel type, air filtration,driving habits, length of trip, vehicle load, engine idle time and so on and on and on....if you do create the appropriate calculation you now have the GM OLM algorythm or similar derivative.

 

[427Z06]

Quote:

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None of those calculations mean anything unless you also calculate things like oil type, ambient air temp,fuel type, air filtration,driving habits, length of trip, vehicle load, engine idle time and so on and on and on....if you do create the appropriate calculation you now have the GM OLM algorythm or similar derivative.

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Very good point made here.

Let me just add that doing a yearly UOA (with a real professional interpretation by someone like Terry), would probably do more than oil selection on increasing the probability of long engine life. If you catch a small coolant leak at say 40K miles and repair it, odds are much greater you'll see 160K-200K miles out of that engine before it fails or runs very poorly.

 

[Steve S]

You left out the most important factor which is luck.I used to work for a forklift dealership . The rental department had approx. 300 of the exact same forklifts with mitsubishi engines .Some used oil some didn't some burned some oil "smell" at 15,000 + hours some didn't smell at 27,000 + hours "I remember the over 27,000 hours for some reason.These were propane using Delo 400 oil and factory branded filters.

 

[jorton]

You used 120k miles MTBF in your example. Can you recalculate your failure percentages using a realistic MTBF so that younger visitors won't be scared off?

 

[LouDawg]

I'd probably use multiple linear regression models if I really wanted to track failure modes. Then I can add or remove how ever many factors/variables I needed to really pinpoint these statistical probabilities.

Or, I could just change the oil at recommended intervals, drive smart, take care of the routine maintenance, and hope/pray for the best.

 

[jbas]

"My current example is a 2005 Ford Duratec 3L engine. Factory spec is MC 5W20 blend with 5K OCI (normal) and 3K OCI (severe duty). Warranty requires 5K OCI minimum...but the concept is the same; better oil with reduction in OCI done in a cost effective manner can extend life, reduce uncertainty, and reduce overall ownership cost."

There are some on here that would argue a 5K OCI would actually produce less engine wear than a 3K OCI. And can back it up with UOAs. (Better filtration as the filter ages might be one reason). Now, if the comparison is between 5K and 15K OCIs, that might produce statistical significance. But I think the A/C, tranny, other parts will wear out first anyway.

 

[Jaybird]

I am seeing that the majority of wear metals are created during the first 5000 miles of a 10k OCI.
I see this on not only my UOA's, but others as well.

If the case is that oil is allowing the majority of wear during the early hours of run, how can we say that we are increasing longevity by shortening the OCI?

This funny little issue is also causing me to take closer looks at the value of a fluid with a higher HT/HS rating, which seems to be a big marketing point these days. (shrug)

 

[toocrazy2yoo]

120K MTBF? C'mon. What failures? We talking internally lubricated, or externals, too? Alternators, starters and the like? 120K?

Man, I must be one lucky puppy. I've been driving them new, used, and ultra-used my whole life (my last car was a 92 Hyundai Elantra with 288,000 miles on the bottom end and it burned no oil, passed the emissions the last month I had it, and never used a drop of coolant. A supposedly low-quality Hyundai). Never an internal engine failure ever, on any vehicle I've ever owned, and every one of em over 200K. I've read countless stories here and elsewhere of 200K routinely achieved on Dino 5K OCI. All makes, all models, Fords included especially patrol cars, which don't exactly live the life of the lazy.

So when you quote MTBF, and I'm assuming you're discussing internally lubricated failures of 120K, well it doesn't hold a lot of water. Cars of the 1970s and earlier, ok, I buy it, but from the fuel-injected era, assuming a reasonable OCI, I don't. 120K is just getting warmed up.

Besides, how realistic is MTBF to 120K on a 2005 DuraTec, anyway? What am I missing here?

 

[Matt89]

Regardless of whether the actual MTBF listed or conclusions drawn were valid, thanks to glenncof for bringing this up for discussion.

My 'takeaway' from this is that manufacturers don't design vehicles and specify maintenance schedules to get owners 'just past the maintenance period' as some conspiracy theorists want to believe. They try to strike a balance between lifespan and cost, with factors like marketing and planned obsolescence thrown in.

My guess is that the actual MTBF of vehicle engines is much higher than 120K. Of course a MTBF target would assume that each vehicle was actually getting the prescribed maintenance LOL

 

[toocrazy2yoo]

Well, the planned obsolescence is also a regulated, built-in issue as the clean air and, I suspect, economy issues will continue to advance. It's been said many times here, no matter how long your engine lasts, the suspension, interior finish, and body deteriorate to the point that you just don't want the car anymore regardless of the condition of the engine itself. Then of course the electrical systems start getting hinky, wiring deteriorates, and there are so many issues other than the engine, the car is junked. They're just not worth it, other than a rare car (the 60's-era muscle cars come to mind among others) or if there's some sentimental value beyond the car itself.

MTBF is an interesting read, but as regards engines, not terribly relevant for the folks here that I can see. We're all maintenance freaks. I tell ya, if I was ever shopping used cars, I'd shop here first. If there's another place on the web you'd be assured of a well-oiled machine, I wanna know where..