It is not recommended to wash an engine while it’s hot. If you wash a hot engine with cold water, you run the risk of damaging parts. Cold water on a hot engine block or manifold can result in it cracking which could ruin your whole engine. Before you wash your engine, the metal parts of the engine should be cool to the touch. It’s okay if the engine is a little warm when you wash it, but going from extreme hot to extreme cold can cause major problems.