Fuel Dilution on Port Injection Hybrid


Thread starter
Jun 5, 2021
Salt Belt
I recently short-tripped a lot and fuel dilation start showing up on UOA. I understand that gasoline in oil won't evaporate when temperature is low, but how does it get into oil in the first place? After all, on a port injection engine, the gas was injected at intake, sucked into the cylinder and then combust, there seems no way for gas getting mixed with oil. Does the fuel dilation mean something is wrong on my car or I'm missing something when think about it? Many thanks.

Ps: pardon my typo, dilution not “dilation” lol
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Oil lubes the pistons. There is always some fuel mixed into the oil in the combustion chamber, and some of that gets pushed back down into the lubrication system. There's always going to be some fuel dilution, it's excessive dilution that is a problem.
When you make short trips, at least two factors work against you regarding fuel getting into the oil, both because the engine is cold. First, it's running richer - more fuel. Second, because the oil is cold, volatiles (like fuel and water) don't evaporate much if at all.

Conversely, when the engine reaches full operating temperature it runs leaner so there's less fuel to begin with, and volatiles in the oil evaporate quickly.

One way fuel gets into the oil is the piston rings aren't perfect. Some combustion gases go past the rings, and some oil is left behind on the cylinder wall.
As mentioned above, the fuel gets past the rings in a number of ways. If combustion gasses and unburned fuel did not get past the rings, oil would stay perfectly clean and we would not need PCV systems.