Understanding Fuel Dilution, Causes and Risks

[RaymondP]

We own two vehicles that if I understand it correctly are prone to fuel dilution of the engine oil as they both feature fuel injection and turbo engines. One is a 2016 BMW 328i (Mobil 1 Euro 0w40). The other is a 2021 Mercedes GLC300 4matic (Mobil 1 ESP 0W30). I change the oil/filter every 7-8,500 miles. Both are driven lowish annual miles, mostly to the local stores and the gym for about 80% of the miles driven. However, there are longer trips, (one to three/four-hour drives,) for the occasional extended excursion or vacation trip. Both vehicles sometimes operate in the 90s in the summer months, as well as well below freezing during the winter months here in Michigan.

I do all my own maintenance whenever possible. So far both vehicles are running well with no issues apart from being a bit "rattly" at start up. Do the longer trips remove or eliminate any potential fuel dilution that may have compromised the oil? Or is this wishful thinking? Am I correct to be concerned about fuel dilution given the above scenarios? My hope is to keep both vehicles for a long time, particularly as vehicle prices continue to rise.

 

[iCurious]

Subjective but on my GLS450 or any Euro vehicles, I use Mobil 1 ESP 0W30 or 0W40 and I change them every 5k miles.

 

[FZ1]

We own two vehicles that if I understand it correctly are prone to fuel dilution of the engine oil as they both feature fuel injection and turbo engines. One is a 2016 BMW 328i (Mobil 1 Euro 0w40). The other is a 2021 Mercedes GLC300 4matic (Mobil 1 ESP 0W30). I change the oil/filter every 7-8,500 miles. Both are driven lowish annual miles, mostly to the local stores and the gym for about 80% of the miles driven. However, there are longer trips, (one to three/four-hour drives,) for the occasional extended excursion or vacation trip. Both vehicles sometimes operate in the 90s in the summer months, as well as well below freezing during the winter months here in Michigan.

I do all my own maintenance whenever possible. So far both vehicles are running well with no issues apart from being a bit "rattly" at start up. Do the longer trips remove or eliminate any potential fuel dilution that may have compromised the oil? Or is this wishful thinking? Am I correct to be concerned about fuel dilution given the above scenarios? My hope is to keep both vehicles for a long time, particularly as vehicle prices continue to rise.

That's what they say. If you wanna find out, do a cheap uoa.

 

[BISCUT]

To me, the engine dictates the OCI and filter change. I have a Outback 3.6 with 220k that has had a life of 7500k OCI's. I like the 7500k mark. I've had an X3 GTDI engine for a year and a half and actually like it. I learned quickly, 5k OCI is the way to go with my engine (N20). They run rather hot and are tough on oil.

 

[Cujet]

To me, the engine dictates the OCI and filter change. I have a Outback 3.6 with 220k that has had a life of 7500k OCI's. I like the 7500k mark. I've had an X3 GTDI engine for a year and a half and actually like it. I learned quickly, 5k OCI is the way to go with my engine (N20). They run rather hot and are tough on oil.

Associated with that is sump volume. Some engines such as my 3.0L Jaguar F-Type hold 8 quarts. That is one of the ways Jaguar was able to justify an 18,000 mile OCI. Complete nonsense of course, but 5000 miles may be more frequent than necessary with high quality oil and a large sump.

 

[fantastic]

Do the longer trips remove or eliminate any potential fuel dilution that may have compromised the oil?

Not really, highway runs are more for burning off any moisture not fuel.

Am I correct to be concerned about fuel dilution given the above scenarios? My hope is to keep both vehicles for a long time, particularly as vehicle prices continue to rise.

Yes, not sure on those specific engines but DI is known to cause more fuel dilution. With the oil & mileage you're using I don't think it'll be too much trouble for you but it's usually best to sample your oil w/GC fuel testing for the data your after.

 

[BISCUT]

Associated with that is sump volume. Some engines such as my 3.0L Jaguar F-Type hold 8 quarts. That is one of the ways Jaguar was able to justify an 18,000 mile OCI. Complete nonsense of course, but 5000 miles may be more frequent than necessary with high quality oil and a large sump.

Correct. My N20 is 5.2 quarts. I'd love a 6-quart capacity!

 

[RaymondP]

Correct. My N20 is 5.2 quarts. I'd love a 6-quart capacity!

I like the 5qt capacity. One container fill and done. No messing with top up and risk of an overfill or underfill. Used oil goes straight into the empty container and both oil and container are recycled. Actual capacity is 5.2 quarts on a dry engine. Filter housing and the engine seem to retain about 0.2 quarts after an hour draining so all good.

 

[Marcozi]

I suggest that you drive the cars at highway+ speed for 30+ miles on a weekly basis. Doing so will get all of the lubricants and coolant up to proper operating temps and burn off most of the fuel dilution.

 

[BISCUT]

I like the 5qt capacity. One container fill and done. No messing with top up and risk of an overfill or underfill. Used oil goes straight into the empty container and both oil and container are recycled. Actual capacity is 5.2 quarts on a dry engine. Filter housing and the engine seem to retain about 0.2 quarts after an hour draining so all good.

I've always had disdain for non-dipstick vehicles, but I have to commend BMW. If I drain oil and change filter and replace with 5qts...that .2 shows up as under "MAX" on the home screen when measured. Add the .2, and she is right on the "MAX" line.

I'd take the minor encumbrance of a 6+ qt sump over the convenience of a 5qt. I'm a little nutty though.

 

[Donald]

The only way to tell if you have fuel dilluion is with a UOA. Gas fuel dilution is bad, diesel less bad.

The only thing you can do when you have fuel dilution is to dump the oil. You may need to do a few UOAs to determine when the % of fuel dillution crosses the line so you know what you can run for a OCI.

 

[Mainia]

It appears you don't have bad fuel dilution, because you would be telling us about your massive gain in oil level. At my worst, I had 1.5Qts of fuel/water condensation in 3,000 miles. My Hyundai rings were jammed, So if you have the issue use Valvoline Restore and Protect and HPL oils are your friend, Can take as high as 4 to 6 4,000+miles oil changes to fix and possibly a tad more if you have a Hyundai/Kia as many have seen it go as high as 7-8 changes to clean the ring lands out.

 

[BHopkins]

To me, the engine dictates the OCI and filter change. I have a Outback 3.6 with 220k that has had a life of 7500k OCI's. I like the 7500k mark. I've had an X3 GTDI engine for a year and a half and actually like it. I learned quickly, 5k OCI is the way to go with my engine (N20). They run rather hot and are tough on oil.

FYI, when you say "7500k", that equals 7,500,000. Perhaps what you mean is 7.5k, which equals 7500.

I like the 5qt capacity. One container fill and done. No messing with top up and risk of an overfill or underfill. Used oil goes straight into the empty container and both oil and container are recycled. Actual capacity is 5.2 quarts on a dry engine. Filter housing and the engine seem to retain about 0.2 quarts after an hour draining so all good.

You may have missed the benefit of a large oil capacity on an engine. Any contamination in the engine, including fuel dilution, becomes less of an issue with a large oil capacity, since the the ratio of contaminant to oil is changed. This means that the oil can go longer between changes, before the fuel dilution starts causing wear issues.

The only way to tell if you have fuel dilluion is with a UOA. Gas fuel dilution is bad, diesel less bad.

The only thing you can do when you have fuel dilution is to dump the oil. You may need to do a few UOAs to determine when the % of fuel dillution crosses the line so you know what you can run for a OCI.

Donald is correct. Collect and submit an oil sample for analysis at the next normal oil change interval, for each of the cars. If the fuel dilution level is high, shorten your oil change interval. Test again until you have found an oci that results in an acceptable % fuel in the oil.

This is a much easier, and really the only practical solution, to fuel dilution, instead of fretting over your driving style, environment, etc.

 

[Donald]

Obviously don't overlook the possibility something could be worn or broken causing fuel dillution.

For example in a Ford PSD a leaking injector or leaking CP4 pump can cause fuel dillution. I am sure there are similar parts in a DI gas car that can cause fuel dillution problems.

 

[Donald]

Let's go back to a Holley 4 barrel. Not any problems with fuel dillution with a 4 barrel.

 

[KrisZ]

If you don’t smell fuel in the oil then realistically you have nothing to worry about.
And since oil and fuel are both petroleum based products, they mix fully and you cannot go for a drive to “burn it off”.

But fuel dilution or not, with lots of short trips, I would stick to 5k mile OCIs.