The washing of engine is to be attributed rather to the cosmetic treatments that the necessary actions for the car maintenance.
The mud on the engine is no danger to the driver and passengers. Technical features of the dirty engine is not that different those of the clear one.
But at the same time, the dirty engine makes it more difficult to control the leaking of mechanical fluids. And if the failures appear, the car owner won’t be able to see them. This can lead to the bigger problems. Mud also impedes the heat transfer, resulting in its overheating of the engine. The clear one is less likely to catch on fire.
You shouldn’t process the engine with the water that has a high pressure, as it can harm the heater, cause corrosion in the generator plug and tumbler. Moreover, you can lose the stickers with the vital information, taped to the engine compartment. After such a rough treatment, some parts can even lose their paint, which again will cause corrosion. You should also avoid compressed air, as it doesn’t solve the problem of dirt removal from the hard-to-reach places.
“The heart” of the car must be cleaned with water (that has the low pressure), using the quality chemical agents. No one domestic detergent can cope with the motor oil.