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How "Permanent" is Fuel Dilution During an OCI? #5233099 10/07/19 03:38 PM
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Mitsu4B11 Offline OP
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Assuming no glaring fuel system issues, and being attributed to cold weather / frequent short tripping, how permanent is any fuel dilution that may occur during an OCI?

To clarify, is oil capable of "reseting itself" after reaching full operating temperature on a longer drive, to burn off any fuel or moisture that may have ended up in the oil, or do parts of fuel combine with the oil permanently to the point where it can not simply be burned off, leading to a thinner cSt (like oil shear) even after reaching operating temps?


'15 Mitsubishi Lancer 2.0L (4B11) // Pennzoil Platinum 0W-20
Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Mitsu4B11] #5233126 10/07/19 04:07 PM
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domer10 Offline
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Longer trips and getting it up to temp does burn off excess fuel built up by a lot of short trips, but not to the extent of it resetting itself completely to when it was first filled...I always make it a point once every few weeks to get out in the rural areas in the winter time for this very reason.


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Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Mitsu4B11] #5233133 10/07/19 04:14 PM
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Pelican Offline
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You have to make sure you reach operating temperature and run for at least 30 min to burn off fuel & water, but it won't reset itself to the original level, no oil does that.

Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Pelican] #5233151 10/07/19 04:28 PM
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paulri Offline
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This is an interesting point--how long do you have to drive, once operating temps have been reached? You are saying that if you drive just long enough to take it to 200F and over, but then stop--that isn't long enough? That you then have to keep on driving? I'm not doubting what you are saying, I don't know, and would like some clarification.

Originally Posted by Pelican
You have to make sure you reach operating temperature and run for at least 30 min to burn off fuel & water, but it won't reset itself to the original level, no oil does that.


2005 Toyota Sienna LE; 163,000 miles
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Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Pelican] #5233152 10/07/19 04:29 PM
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4WD Offline
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Another question is how much volume would it take to thin the oil … most OCI's don't show a detectable fluid gain in an engine known to not consume oil - so there can be other reasons for an oil to be thinner at the end of a normal run where oxidation has not offset any thinning.

Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Mitsu4B11] #5233190 10/07/19 05:23 PM
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Brian553 Offline
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The gasoline is composed of a variety of constituents (or fractions). Some fractions of the gasoline can be volatilized easily and be burned off once they pass by the piston rings, or if they make their way through the pcv valve.

Not all fractions of the gasoline will be burned off. Some heavier fraction have a stronger pull to remain in the oil. As the oil is used in service, it's viscosity will gradually drift.

To my knowledge, this is simply part of the lubricant's design parameters, that it would bolster protection in anticipation of this occuring in-service.

I have not found out how a lubricant is designed different to maintain this protection.


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Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Mitsu4B11] #5233224 10/07/19 06:10 PM
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doyall Offline
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"The boiling point of gasoline ranges between 104 and 392 degrees Fahrenheit. The wide range of boiling points is due to the many different blends of components available to provide different characteristics such as higher octane, lower fuel deposits and overall volatility.

The combination of chemicals in gasoline to produce different octane levels is one factor that influences the boiling point. A fuel's octane influences its tendency to join in reactions prior to combustion, and increasing octane means adding agents that minimize those reactions. The specific additive used to boost octane, reduce harmful emissions or produce other effects pushes the boiling point higher or lower."

https://www.reference.com/science/boiling-point-gasoline-4ee47a3b379ed4c2


This suggests to me that the gasoline constituents that will not boil (vaporize) at operating temperature of the oil will be unlikely if not impossible to be evacuated through vaporization.


From my experiences with dozens of oil analyses on multiple engines from different manufacturers, I have yet to have any significant "burn-off" of fuel dilution after 200, even 300 miles of continual usage. (Some UOA's are posted, most are not.) My take, once it is there it is there for the duration. FWIW, my wife's car ('17 3.6L Pentastar) has an oil temperature read out. It typically takes 20 - 30 minutes of at-speed operation before the oil gets to operating temperature (generally around 195°F). At that temperature it is going to take a long time for the fuel that got into the oil at a cold start that will vaporize, to vaporize.

Last edited by doyall; 10/07/19 06:12 PM.
Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Mitsu4B11] #5233239 10/07/19 06:28 PM
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Originally Posted by Mitsu4B11
Assuming no glaring fuel system issues, and being attributed to cold weather / frequent short tripping, how permanent is any fuel dilution that may occur during an OCI?

To clarify, is oil capable of "reseting itself" after reaching full operating temperature on a longer drive, to burn off any fuel or moisture that may have ended up in the oil, or do parts of fuel combine with the oil permanently to the point where it can not simply be burned off, leading to a thinner cSt (like oil shear) even after reaching operating temps?



No, part of the fuel will volatile easily, but some of it will remain. Some will,oxidize and create solids that the dispersant package is designed to deal with. .

Basically oil formulations are designed with this use in mind. Unless you visit BITOG you probably would never think to “smell your dipstick”
That is why natural gas engine oil, does not require the tbn numbers as a gasoline oil, the demand is different.


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Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Pelican] #5233240 10/07/19 06:29 PM
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Lowflyer Offline
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Originally Posted by Pelican
You have to make sure you reach operating temperature and run for at least 30 min to burn off fuel & water
Yeno (yes and no wink2)

You are right, but you cant burn off the (bio)ethanol part of fuel (if its present). Sadly.

Re: How "Permanent" is Fuel Dilution During an OCI? [Re: paulri] #5233243 10/07/19 06:30 PM
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Pelican Offline
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Originally Posted by paulri
This is an interesting point--how long do you have to drive, once operating temps have been reached? You are saying that if you drive just long enough to take it to 200F and over, but then stop--that isn't long enough? That you then have to keep on driving? I'm not doubting what you are saying, I don't know, and would like some clarification.

Originally Posted by Pelican
You have to make sure you reach operating temperature and run for at least 30 min to burn off fuel & water, but it won't reset itself to the original level, no oil does that.



Yes I heard that from a mechanical engineer and it makes sense. Think of reducing any liquid by heat, you have to reach the right temp and then keep at that temp for as long as it takes to obtain the desired results. It may be that in some cases less than 30 min will suffice, I think that the 30 min period was used as a safe time to be sure.

Last edited by Pelican; 10/07/19 06:32 PM.
Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Mitsu4B11] #5233244 10/07/19 06:31 PM
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benjy Offline
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a good reason for 6 month change intervals and NO xxw20 oils that are thin from the go and no direct injection, which is one reason i bought my frontier which is still NOT DI

Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Mitsu4B11] #5233264 10/07/19 06:49 PM
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paulri Offline
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I have seen threads on driving before getting a UOA--the idea being you would warm up the oil & mix it up before taking the sample. I guess driving for 20 minutes or so would not burn off much fuel in the oil--I was thinking that it might not be good to drive too long before a sample was taken, because that might remove the evidence of fuel in the oil. But if what I'm reading here in this thread is correct, then that won't happen.


2005 Toyota Sienna LE; 163,000 miles
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Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Mitsu4B11] #5233274 10/07/19 06:57 PM
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pitzel Offline
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Transient. A 10k mile oil change in a DI engine will consume several hundred gallons, 400-500 gallons if not more over that interval. Only a pint, maybe a quart of that fuel at best will end up in the oil and persist. So obviously its a very dynamic, not a cumulative process in most cases.

An engine really should be sampled when running UOA-wise, either on-load or immediately after load is removed. Of course nobody does that (and a turbo engine should never be turned off immediately after coming off-load), which is why fuel dilution numbers are often exaggerated.

The viscosity thinning that's experienced in the 'startup' oil may very well reduce wear.


Change your thinking...not your oil!
Re: How "Permanent" is Fuel Dilution During an OCI? [Re: pitzel] #5233541 10/08/19 04:50 AM
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PWMDMD Offline
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Originally Posted by pitzel
Transient. A 10k mile oil change in a DI engine will consume several hundred gallons, 400-500 gallons if not more over that interval. Only a pint, maybe a quart of that fuel at best will end up in the oil and persist. So obviously its a very dynamic, not a cumulative process in most cases.

An engine really should be sampled when running UOA-wise, either on-load or immediately after load is removed. Of course nobody does that (and a turbo engine should never be turned off immediately after coming off-load), which is why fuel dilution numbers are often exaggerated.

The viscosity thinning that's experienced in the 'startup' oil may very well reduce wear.


I do not understand your rationale in either the first or second paragraph. The process can be both dynamic and cumulative and that’s likely the case. The more you drive the more gas enters the oil. During operating temps some fractions are volatilized and removed from the oil and some are not. The further you drive the more fuel fractions are not volatilized and the greater the fuel dilution. It’s both dynamic and cumulative.

Last edited by PWMDMD; 10/08/19 04:53 AM.

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Re: How "Permanent" is Fuel Dilution During an OCI? [Re: Mitsu4B11] #5233548 10/08/19 05:10 AM
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I remember reading something a long time ago about just how much fuel 'washes' through the oil in normal use - it was litres. Yes, you lose most of the fuel when the oil heats up. No, you don't lose all of it (as pointed out above about heavier fractions etc). Not to mention some of the species coming from the fuel may have a chemical attraction to molecules in the oil, so will stay there.


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