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Water in the crankcase is usually caused by trips to the grocery store, prolonged idling, other short trips where both the water and oil temperature do not reach a high enough temperature for a long enough time. Until the engine block temperature along with the oil temperature reach higher than 180 degrees, the "blow by", air leaking past the piston rings will lose it's moisture as it makes contact with the cooler boundary layer next to the cylinder walls. As the moisture settles into the bottom of the crankcase, a small amount is absorbed into the oil but the rest remains in little droplets containing acids and other contamination. The higher the humidity, the more moisture will condense. With oil temperatures less than the mid-180 degree mark, the oil will not evaporate and may actually increase in concentration with continued driving practices. The breather cap is cooler than the engine interior so moisture passing through the cap will condense and form the nasty gray sludge seen there.

To avoid this, the engine coolant thermostat should be 180 degrees and the engine should be run at a high enough load so the oil will also exceed 180 degrees for a long enough period for the moisture to evaporate and pass out of the engine.  Cummins, Caterpillar, etc all say the same thing-- if you can't drive 25+ highway miles, don't start the engine.

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