Recently, I was contacted by a reader who thought his oil might be “swelling,” because he got dipstick readings above the full mark immediately before changing oil. He wanted to know if oil could possibly “swell.” Many modern low oil-consumption diesels could easily give the operator the impression that oil was swelling in service.
The reader was told that diesels with high EGR rates collected so much soot in the oil that it caused significant expansion. During a typical oil change interval, the oil in the sump may collect as much as 4% to 5% soot, primarily in P&D service. However, soot particles in modern oils are very small (less than one micron), so they don’t add significant volume to the crankcase oil. The slight increase in volume would be very difficult to read on a dipstick.
The major reason for crankcase oil to “swell” or expand in service is fuel dilution. Diesel fuel is much more critical than gasoline in this regard. A fully warmed-up gasoline crankcase will reach temperatures as high as 300 degrees F, and gasoline oils are much thinner (typically SAE 5W-20 grade) than diesel oils. Gasoline (and low viscosity oils) are mixtures of many compounds from relatively heavy compounds to very light compounds (called light ends) to assist in starting a cold engine. Subsequently, hot automotive crankcases evaporate many of the lighter ends in gasoline engine oils. This prevents swelling, and it is why there is a NOACK volatility specification for modern passenger car oils.
Diesel fuel is significantly less volatile than gasoline since the range of compounds in diesel fuels tends to be much narrower with few “light ends.” In addition, fully warmed-up diesel engine crankcases operate at much lower temperatures. The majority of diesel oils tend to be SAE 15W-40 viscosity grade, which is thicker, therefore less volatile, than gasoline engine oils. As a result, very little of the diesel fuel in the crankcase is ever driven off (evaporated), and fuel dilution can easily be observed when checking with a dipstick. It is possible to observe diesel fuel dilution levels as high as 7% to 10% in P&D operation.
Coolant dilution also can cause an apparent increase in the quantity of oil in the crankcase, but most of today’s diesels have addressed that problem with improved cylinder liner sealing. If you should see coolant in the oil when you are changing it, and it is pretty obvious, oil expansion is perhaps the least of your problems. Get your engine into the shop— quickly!
But the reader’s question made me think about some of the other things that could give vehicle operators the impression they are “making oil” in the crankcase. For example, you should always check the oil on level ground. Dipsticks are supposed to be placed in the middle of the crankcase, but vehicles setting at different angles have different amounts of oil draining back from the top end of the engine due to the location of the drain-back holes in the cylinder heads.
You should also allow the oil a few minutes or so to drain into the pan before checking the oil level. This is particularly critical in colder engines with thicker oils. If you changed your oil and then checked the level immediately before the engine was allowed to come to operating temperature, you would have much less “apparent” oil than if you waited a minute or two for the hot oil to drain back from a fully warmed-up engine. I’ve seen 10% to 15% differences in how much oil is retained topside on an engine during oil changes.
If your oil is “swelling,” make sure your dipstick checking procedures are correct, then look for fuel or coolant dilution.
