P-chem doctors: What is the chemistry of gasoline in oil?

Not open for further replies.
Jan 1, 2003
I think that we generally agree that lots of idling, especially in cold weather, loads the oil up with fuel.

What happens to that fuel?
Is gasoline fully miscible?
Are there reactions between the two fluids that cause precipitates or emulsification?
If the oil heats up to operating temperature, will all of the fuel evaporate and exit the crankcase as blowby or otherwise?
Is there permanent degradation of the oil, or will it recover its viscosity and FM/AW properties?

Are there other questions that I have negelected to conceive that you would care to answer?
Yes, gasoline is fully miscible with motor oil, though a mixture left standing long enough might result in phase separation due to their different specific gravities. Once warmed, the gasoline will evaporate and be purged through the engine by the PCV sytem unless some malfunction is constantly supplying more gasoline into the oil other than initial cold-start considerations.

When I was in college, my roommate showed up one Sunday night at the dorm with a can of engine treatment that proclaimed an "Amazing, All Natural, Breakthrough Lubrication Discovery" and "This product will immediately increase your engine's power as you watch in disbelief!". He asked me to put it in since he was totally lost around mechnicals. He restarted the still warm engine on his '61 Chevy, I removed the oil filler/breather cap, shook the can vigourously per instructions, and poured the contents in. Almost immediatley the engine idle speed increased about 50 RPM and he shouted, "Wow! That stuff really works." I noted an unmistakable odor of gasoline, and, sure enough, it was emmaniting from the now empty can. Within a minute or so the idle speed tapered off again. I just kept my mouth shut. If paying $12.00 plus shpg. to J.C. Whitney for an 8 oz. can of questionable pedigree mouse droppings in a gasoline carrier was all it took to make him happy, who was I to rain on his parade? But the product's advertising was correct in one respect. What I witnessed, indeed, left me in total disbelief.
I think the chemistry of the oil has been affected by fuel while in solution, and it makes minor changes in the oil.

My belief is that fuel still in solution gets partially cooked on the cylinder wall and these partially cooked hydrocarbons contribute greatly to cold weather oil degradation in the form of oil darkening sludge products.

[ April 19, 2003, 02:54 PM: Message edited by: MolaKule ]
I agree with MoleKule. Fuel contamination lowers the flash point of the oil, even at low levels. I think that coking in the cylinders is the source of particulates (which darkens the oil and lead to sludge formation, etc.) The UOAs from my old Honda, which is driven exclusively for a short commute, always show high particulates yet there is reserve oxidation capacity. This is the result of chronic fuel contamination. Only samples taken in the winter show fuel contamination. So it would appear that sump temperatures get high enough to vaporize the fuel in the summer, but not in the winter. But the particulates stick around.
Persistent fuel contamination accelerates accumulation of oxidation products and particulates in the oil. Viscosity decreases as a function of % fuel with an associated increase in wear rates. It usually takes more than 1% fuel in the oil to see significant viscosity effects.
I had some experience with this in the last 6 months. I put a oil seperator/filter on the PCV system of my '87 Buick Grand National sometime last summer. Everything seemed good for a while. I was getting about an ounce or two of oil in the collector every 1,000 miles. It was nice to know that crud wasn't coating the intake system and being burned in the cylinders like it had been. Then came winter temps.

I checked the PCV collector with about 40 miles on a recent oil change last November. There was probably close to 2oz of raw gasoline in there
I thought my engine was hosed. So I watched it over the cold winter months on the rare, dry days that I drove it. It was the same thing as far as what was being collected. Obviously I was super worried but after researching on here and other enthusiast boards for my particular car, I noticed this seemed like it may very well be typical for cold weather. Difference being that most people aren't able to see the amount of activity going on with blow by and the PCV system without a collector/filter.

During the winter, on warm days(50f+), there was just trace amounts of gasoline in there and now that it's even warmer the gas collected is gone completely.

Keep in mind, the car always drove and idled normally, doesn't use any noticeable amount of oil at all, the oil level never showed going up or down and the oil still looks nearly new on the dipstick. As a note, this car has darkened the oil faster than other cars I've had. Usually it's fairly dark by 1,500 miles.

I also had a thermostat that was apparently running colder than it should. It's a 180* thermostat but when I put the car on the computer in past winters it would show temps in the 150's In the summer it always ran where it should so with the 500 or so miles I put on it in the winter I didn't bother changing the stat. I did that last week
so I'll see if there's any improvement with the blow by next winter.

Anyway, this was an eye opening experience regarding blow by, fuel dillution and how active a PCV system can be in cold temps etc.
Not open for further replies.