I can't quite wrap my head around GDI engines and fuel dilution of the engine oil

Joined
Jan 5, 2023
Messages
2
With gasoline as volatile as it is, and engine operating temperatures as high as they are, and PCV systems doing their thing, how does gasoline manage to dilute the engine oil? It seems like it would get vaporized as soon as the engine warmed up. How come it doesn't? Does it combine with the oil forming a different solution?
 
With gasoline as volatile as it is, and engine operating temperatures as high as they are, and PCV systems doing their thing, how does gasoline manage to dilute the engine oil? It seems like it would get vaporized as soon as the engine warmed up. How come it doesn't? Does it combine with the oil forming a different solution?
IF the oil can get, and stay, hot enough to boil it off, it will dissipate, but often that's not the case.
 
With gasoline as volatile as it is, and engine operating temperatures as high as they are, and PCV systems doing their thing, how does gasoline manage to dilute the engine oil? It seems like it would get vaporized as soon as the engine warmed up. How come it doesn't? Does it combine with the oil forming a different solution?
On non di engines the air feul mixture has more time to vaporize in the intake valve area. The poor/incomplete combustion also produces soot.
 
It does just not fast enough, Takes time for the oil to heat up and to keep it hot and many people do short trips so that becomes unfeasible. But the dilution isn't the worst unless it's like the early honda 1.5t's which would add an extreme amount of fuel to the oil. But around 5% dilution is acceptable and with oils being designed to withstand the effects of gasoline it's becoming a smaller issue. It can be better combated by just bumping up a grade if you've got a gdi and short trip it a lot. Doing an occasional long drive helps too. Sometimes an injector will just plain be defective.
 
Many of the modern GDI engines come with higher compression. My Mazda has 13:1.

Also, the fuel injectors do not just inject at TDC. The computer tells them when to inject fuel and in some cases they inject multiple times during a cycle. When the engine is cold they run very rich in Order to expedite warm up and for the catalysts. That’s how cars that run short trips and especially in colder weather can develop fuel dilution.

GDI has been around for quite some time. Mitsubishi was putting them in vehicles thirty years ago at least.
 
When I look at used engine oil reports that folks post from their GDI vehicles, it's evident that fuel dilution continues to increase with mileage. And that's one of the main reasons it's recommended that GDI owners change oil by about 5K miles or so, regardless of what the owner's manual or engine's oil change indicator says.

I guess it doesn't make sense to me that just a single hour of highway driving wouldn't cook all the gasoline out of the engine oil, reducing the dilution amount to zero each time you made that one hour highway trip. Heck... gasoline has a flash point of something like -45˚ F. But it somehow manages to stay in the oil. Interesting...
 
I guess it doesn't make sense to me that just a single hour of highway driving wouldn't cook all the gasoline out of the engine oil, reducing the dilution amount to zero each time you made that one hour highway trip. Heck... gasoline has a flash point of something like -45˚ F. But it somehow manages to stay in the oil. Interesting...


If you are old enough to remember carburetors they also had a tendency to do the same thing, especially if the choke was used a lot or left pulled out or the carburetor was in need of adjusting.
 
Many of the modern GDI engines come with higher compression. My Mazda has 13:1.

Also, the fuel injectors do not just inject at TDC. The computer tells them when to inject fuel and in some cases they inject multiple times during a cycle. When the engine is cold they run very rich in Order to expedite warm up and for the catalysts. That’s how cars that run short trips and especially in colder weather can develop fuel dilution.

GDI has been around for quite some time. Mitsubishi was putting them in vehicles thirty years ago at least.
They also run richer to keep the NOX down with the higher compression.
 
img_20211105_213523638-jpg.76623

img_20211105_214308183-jpg.76625
 
And fuel is injected at a couple thousand psi; what makes it out of the piston bowl and onto the cylinder walls as a liquid is much more likely than a vapor to make it past the piston rings and into the crankcase. As stated, the proximity to where the combustion takes place compared to a port injector reduces the chance of “real” vaporization.
 
It takes me 20 miles on the highway before I reach 180F on the oil temp. Short trippers own a nightmare if GDI only. I don't know at what temp the evap rate is higher than the refill rate. I don't know how long it will take for the evap rate to overcome previous dilution filling. Obviously, the automaker wasn't prepared for the US consumer.

Looks like the R&D was a complete failure. Informed consumers eventually figured out what was happening.

Modern manufacturing! Trust your automaker! Trust your dealer! I've heard all the cliches on too many forums with too many brands/models... their pitiful clueless blind trust existence!

I also know that some cam driven HPFP's leak. They might be dripping into the oil. Scary thought. Quality control or expectations of the HPFP might also be excessive.
 
It takes me 20 miles on the highway before I reach 180F on the oil temp. Short trippers own a nightmare if GDI only. I don't know at what temp the evap rate is higher than the refill rate. I don't know how long it will take for the evap rate to overcome previous dilution filling. Obviously, the automaker wasn't prepared for the US consumer.

Looks like the R&D was a complete failure. Informed consumers eventually figured out what was happening.

Modern manufacturing! Trust your automaker! Trust your dealer! I've heard all the cliches on too many forums with too many brands/models... their pitiful clueless blind trust existence!

I also know that some cam driven HPFP's leak. They might be dripping into the oil. Scary thought. Quality control or expectations of the HPFP might also be excessive.
I’m surprised you didn’t jump into the UOA thread the other day about fuel dilution. This description may have actually made some sense to that OP, but probably not. 👍🏻
 
With gasoline as volatile as it is, and engine operating temperatures as high as they are, and PCV systems doing their thing, how does gasoline manage to dilute the engine oil? It seems like it would get vaporized as soon as the engine warmed up. ...
Commercial gasoline is a mixture. Some components are more volatile than others.
 
I'm not a tribologist and dont want to even pretend like I am one, but I have worked with a few over the years. One of the guys in aviation tech school, twenty or so years back, was talking about fuel dilution and for whatever reason what he said stuck with me..... He would say "If you smell fuel in the oil, get the oil out of there. Even if you could evaporate all the components of the fuel out of the oil, which you'll never be able to do in the confines of your average engine, whatever VII's and PPD's and other additives were used when the oil was formulated have been chemically altered, so add all the heat you want, without re-refining that oil and starting over, you wont get the original oil formulation back to where it was before the gas was in there. An Italian tuneup wont fix this issue. Get it out of there and replace it with new oil". Right or wrong, thats the advice I was given and what I've followed regarding fuel dilution. He gave us some lab reports to back this up and I know I saved them in a box somewhere, I really hope I can find those one of these days...

I'm just a dude on the internet, so take that with a grain of salt. I dont have the proper credentials to yay or nay what he said.
 
It takes me 20 miles on the highway before I reach 180F on the oil temp. Short trippers own a nightmare if GDI only. I don't know at what temp the evap rate is higher than the refill rate. I don't know how long it will take for the evap rate to overcome previous dilution filling. Obviously, the automaker wasn't prepared for the US consumer.

Looks like the R&D was a complete failure. Informed consumers eventually figured out what was happening.

Modern manufacturing! Trust your automaker! Trust your dealer! I've heard all the cliches on too many forums with too many brands/models... their pitiful clueless blind trust existence!

I also know that some cam driven HPFP's leak. They might be dripping into the oil. Scary thought. Quality control or expectations of the HPFP might also be excessive.
DI has been used for essentially 20 years. Please do tell were are all these failed DI engines due to fuel dilution?
 
With gasoline as volatile as it is, and engine operating temperatures as high as they are, and PCV systems doing their thing, how does gasoline manage to dilute the engine oil? It seems like it would get vaporized as soon as the engine warmed up. How come it doesn't? Does it combine with the oil forming a different solution?
Low tension rings and short tripping allow unburnt fuel to get into the crankcase and accumulate. The fuel will burn off once the vehicle reaches operating temperature. The amount of fuel dilution is engine dependent. Some TGDI engines experiences this and some do not but people love to paint with a broad brush.


Note: To those who want to chime in and say that a TGDI won't last 350k miles. The reality is that nobody cares, seriously, nobody cares. Go scream at the neighbors dog or something.
 
Low tension rings and short tripping allow unburnt fuel to get into the crankcase and accumulate. The fuel will burn off once the vehicle reaches operating temperature. The amount of fuel dilution is engine dependent. Some TGDI engines experiences this and some do not but people love to paint with a broad brush.
Some of it will but not all of it. Plus the presence of the fuel can cause permanent degradation of the viscosity index improvers used in the oil. Gasoline in the oil is not as benign as people like to believe.
 
Back
Top