I have never washed the engine compartment but I am planning to sell my car soon so I took it to https://atfulldrive.com/best-foam-cannon last week and they did it for me. Now it looks so good that I am having second thoughts about selling. I have a friend who is a mechanic and he suggested not to wash the engine compartment when trying to sell as the potential buyer could think that you are trying to hide leaks from him. I see the logic of it but considering that people who buy second cars are not all mechanics I don't think that a dirty engine can attract many buyers.