Has fuel diluted your engine oil? Is this a real thing in modern engines? Is it wreaking havoc down there? How would you even know, and what can you do about it?
One viewer recently asked me to solve his alleged vehicular problem:
"I have a problem with my 2015 Mazda 3 (2.0L GDI). Randomly I took an oil sample analysis and the report showed that there is a 'minor fuel dilution with engine oil'. So I contacted the Mazda dealership but they said it seems okay. They suggested I take another oil test after 1000km."
OIL DILUTION: BACKGROUND BRIEFING
Oil dilution is a real thing. An operational reality. It’s more of a factor now than ever before. Here’s the background briefing on that.
Especially in a cold engine running rich, fuel can condense on the cylinder walls, where it sort of dissolves into the thin film of lubricating oil that’s there. The oil, only a few microns thick, is trying to maintain a death grip on the fine scuff marks honed into the bore. And the fuel’s not helping.
Here's what Wikipedia says about crankcase dilution >>
When the rings sweep past, downwards, they scrape it off. The oil, diluted with fuel, is flung into the crankcase. This is happening about 8 times a second for every 1000rpm - per cylinder - so you don’t need all that much fuel per combustion event to get mixed up in this way, for it to really add up over time. And the result is: Rather a lot of fuel can end up mixed with the oil. More detail on the phenomenon in this report on oil dilution >> from Total.
It’s not good - not desirable, anyway - but it is known about, and it is taken into account in R&D at both oil companies and carmakers - we’ll get to that.
Basically, fuel, petrol (gasoline) is to lubrication what Idi Amin was to humanitarianism. The function of oil is to maintain a tough, thin, slippery film between the critical metal interfaces in your engine - on the bores, in the bearings, the cam lobes and valve guides, the timing chain - all the expensive stuff that moves really, really fast, with great precision.
Metal-on-metal contact is the enemy of longevity, and oil is the cure. Fuel in the oil degrades the effectiveness of the oil.
Get a great price on engine oil at Sparesbox >>
MODERN ENGINES ARE SUSCEPTIBLE
Direct injection engines - where the fuel is pumped at extreme pressures directly into the chamber - are more susceptible to oil dilution. Engines that do lots of cold starts and lots of stop-start driving are more susceptible. Lots of heavy acceleration: More susceptible.
And then there’s the service interval, which on low-mileage cars is often 12 months now. 12 months of scraping fuel-diluted oil into the sump, as opposed to six months - or even three (in the olden days).
Carmakers really cannot win here. Direct injection is a huge benefit for fuel efficiency and all-round performance. Oil dilution is a negative feedback effect. However, you are out of touch with reality if you draw the conclusion that direct injection is shit because: oil dilution. It’s manageable negative feedback.
You are likewise dissociated from the real world if you think 12-month service intervals are a mistake. A carmaker would be crucified for going back to six from 12. Oil dilution is a real operational thing that needs to be managed. It’s generally not a defect - although there have been examples of engines with excessive oil dilution.
IS IT EXCESSIVE?
When Mazda first introduced the CX-5 diesel and Mazda3 diesel - same engine - it had excessive oil dilution. They fixed it with a software update and a minor hardware change.
The question continued:
"I only driven 50k but how come I got a minor fuel dilution problem?"
Asked and answered I think. Most likely correct answer there: All engines get fuel dilution to some degree. If you do lots of cold starts, short trips, stop-start driving, it’ll be more significant than if you drive regularly on a free-flowing freeway. Because that’s how this works.
As for what the dealer did:
"I think the Mazda dealer checked the fuel system and crankcase only through OBD2 scanner or electrical system. Will an OBD2 scanner or other electric tool help a mechanic identify the fuel dilution with oil?"
I think, because of the service campaign to repair early CX-5s and Mazda3 diesels, Mazda dealers, in particular, would be pretty well versed in identifying problematic oil dilution. The most common differential diagnosis for non-trivial oil dilution is an increase in oil level.
REPAIRS? ARE WE FXING THIS?
For this reason, some manufacturers have started putting a third mark on the dipstick to indicate ‘over-full’.
In any case, if you change the oil and replace with the specified volume, and then the level climbs beyond the ‘maximum’ level on the dipstick - I’d be doing my best Tom Hanks and telling Houston you’ve just had a main bus B undervolt. Apollo 13 reference >>
Then there's the repairs - if any:
"I'm still in a warranty period (8 months) but I think there is a chance that Mazda Australia/dealer do not want to repair under warranty because at the moment my engine runs perfectly fine. What can I do in this case? I am pretty sure my engine will die earlier than others but they cant repair my vehicle after the warranty period."
This is wrong on many levels. Firstly, the oil analysis you commissioned when you should have been pecker-adjusting like the rest of us, described the degree of dilution as ‘minor’. I think ‘trivial’ would work just as well in this context.
All engines incur some oil dilution - you could test every engine on the road: all positive. Therefore this is a non-defect. Non-defects don’t demand repairs.
Your engine probably will die sooner than others - if you do many cold starts and lots of short trips. All engines die earlier when operated in this way. Just look at taxis - which never go cold, and therefore last several times longer than ordinary cars.
On repairing after the warranty period: All manufacturers are required to guarantee reasonable durability of their products. This is a legislated consumer guarantee. It is entirely independent of the warranty. In other words, the product has to conform to the durability expectations of a reasonable person.
This takes into account the cost of the product, and the context of its operation. So, if you’re four years and 100,000 kays down the track and the engine goes 100 per cent Osama Bin Laden, there’s a case you could make that this reasonable durability standard has not been met, in which case, Mazda would be legally obliged to fix it free of charge (of course this refers only to consumer law here in ‘Straya).
WHAT YOU CAN DO
My general advice here is that the people who designed the engine know about oil dilution. The people who designed the oil know about dilution. Dilution is a thing that gets managed in modern engines. The formulation of the oil is such that it provides adequate protection when diluted up to a point.
The service interval is likewise selected so that dilution doesn’t spiral out of control. This is, in part, why there’s a time component to the service interval, as well as a distance component.
The other thing you can do to manage the dilution is either go on long drives semi-regularly - a couple of hours with lots of lean burning and sustained optimal temperature operation.
That’s gunna help purify the engine oil, in the manner of burning a witch at the stake, with the notable exception being is evaporates out all the volatile fuel components. But apart from that, just like witch-burning.
Alternatively, just change the oil more frequently if you only do short trips. If engine longevity is important to you, you need to realise that a harsh operating environment for oil is not what you might imagine. Get a great price on engine oil at Sparesbox >>
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HARSH OPERATING CONDITIONS
One of the harshest things you can do to oil is cold start, drive to the station (10 minutes), catch the train to work, and drive back 10 hours later, repeat. Five days a week. That’s like a month at the Hellfire Club, for oil. So just change it more often. Twice as often. And the filter. Pretty simple fix.
I didn’t just make that up. Many owner’s manuals recommend changing the oil more frequently in harsh operating environments or as advised by the dealer. Unfortunately, it’s not routinely communicated that driving like grandma to the shops constitutes a harsh operational environment for your engine.
(I think carmakers are afraid that no matter how creatively they craft this advice it will still be perceived in some camps as if they are merely smearing icing sugar upon a bullshit birthday cake. It often doesn’t occur to them that up-front honesty using actual facts would be an effective messaging tool.)
You should check the oil level - once a fortnight or every second time you re-fuel. In the domain of ideas, this is a good one. If the oil level climbs over the full mark, don’t hesitate to get professional advice. The oil won’t be doing it’s engine life-preserving voodoo if that happens - and that’s bad.
Some dilution is normal, but excessive dilution is probably a symptom of a bigger problem - perhaps a serious fuel system or inlet air problem where your engine is over-fuelling. Faulty MAF sensor; cracked inlet air pipe (if the car is turbocharged). And you’ll need to get that sorted.
Minor oil dilution is, however, completely normal. Don’t stress about it.