When the engine is cold, fuel and water from the exhaust gas may condense in the intake system and in the cylinders making its way into the oil prior to combustion. When you first start your car, the ECU is in a “cold start” and “cat heating” mode. During this time, the ecu tells the injectors to spray extra fuel into the cylinders to assist in warm-up of both the engine and the cats. At the end of this cold start phase, the engine oil contains about 15% fuel and water. Fuel and water will continue to mix with oil at temperatures below 122f and start to evaporate out of the oil above this temp. This is why it is so important to fully warm your car up before beating on it. The oil can’t do its job if its full of water and gas.
Ethanol/e85 requires 30% more fuel per HP than gasoline…even at idle. Condensation of e85 causes additional fuel consumption as well. Because of this, during cold-start, fuel mixing into the engine oil is significantly higher in a vehicle running on ethanol vs regular 91/93. In cars running e85, as much as 2/3 of a cup of additional fuel can get in the engine oil during the cold start phase. Unlike gasoline, ethanol does not have a boiling point curve but a fixed boiling point. The boiling point of ethanol is 172f whereas the boiling point of gasoline can be anywhere from 104f to 392f. This is purely due to the chemical makeup of ethanol. Since ethanol is corrosive and acts like a detergent (or degreaser—think of oil as the grease being broken down), it is especially important to let your car fully warm up to ensure all of the fuel is evaporated out of the oil.
During start up, an ADDITIONAL amount of fuel is injected into the engine to provide for the additional volumetric demand and additional start enrichment of the air-fuel mixture required to get the engine running. During this time, the extra fuel, which is not directly involved in the combustion process, comes into contact with engine components that have not yet reached 172f and re-condenses and is either expelled by the exhaust or bypasses the piston rings and is mixed into the engine oil. This process continues until all engine components in the combustion chamber have exceeded the boiling point of the fuel that is being injected into it. After this point, more ethanol is extracted from the oil than is introduced. Fuel mixing into the oil can be even higher if the car is frequently driven short distances. The oil never gets hot enough to burn all of the water and fuel out before more is introduced. This can cause accelerated wear and tear on the engine, especially when beating on it like this.
Once the engine oil reaches 172f, the ethanol evaporates completely. The water and gasoline will continue to evaporate as the oil temps climb and you continue driving. This evaporated fuel, water, and ethanol is now sent into the cylinders via the crankcase breather/PCV…or your catch can. This is why your catch can oil will usually look like chocolate milk. It is not a sign of a bad head gasket, just the mixed fuel, oil, and water vapors pulled out of the crank case(the area under the pistons) and re-condensing and pooling in the catch can instead of being burned up in the cylinders. The catch cans primary job is to catch the oil vapor before it gets sucked into the engine coating the intake manifold with oil and burning up and caking onto your valves and pistons. All of which can lead to pre-ignition/detonation(knock retard) and possibly to engine damage over time.
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