Frontal impact vehicle is there fan motor touches the oil cooler,oil cooler slight impact is there, in coolant tank oil and coolant mixed, what are the reasons to mix the coolant and oil in this situation.
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I take it from the wording, English is not your first language. To answer you outright, there is no reason to mix coolant and oil, under no situation where a vehicle is concerned.
If you are actually asking why oil would be fed through something containing coolant (radiator or such), the reason would be simple in that it is cooling the oil. By cooling the oil, the oil will stay viable longer as well as it will not want to build up sludge as much. It will also be able to cool the engine better, which means the engine itself will last longer. Using an oil cooler works especially well for supercharged or turbo-charged engines.
I take it via the accident, the oil cooler was damaged in some way and it is now allowing oil to flow freely into the coolant? If so, about the only thing you can do is replace the oil cooler. You could possibly pick up a used one from an auto recycler for a lot cheaper than you could purchase one new.
It's not unusual for a second radiator to be present behind the main radiator, often to cool the transmission fluid. If these fluids are mixing (water and transmission fluid), then both radiators have been damaged and are leaking their fluids together. Both radiators will need to be removed and repaired.
Sometimes coolant will appear in engine oil because the head gasket has 'blown' or broken, allowing coolant to enter the cylinder or oil to enter the water jacket. The repair is to remove the head and replace the head gasket.
Sometimes the cause for mixed fluids is a cracked block, in which coolant and/or oil escapes the engine through the crack. The repair is to replace the block because a cracked block cannot be repaired. Normally you'll choose to replace the engine, since it's easier to swap the engine than disassemble an engine to swap its block. Most people would just buy another car.
If your car HAS been in an accident, I would first try to identify the two fluids that are mixed. Then I would check first for a cracked block, then check for broken radiators. The third possibility, a blown head gasket, is unlikely to be caused by an accident.