I am getting ready to put my 98 Grand Prix up for sale. I've detailed the inside and washed/waxed the outside. When I have stopped at a self-service car wash, I see the tire/engine cleaner option. Is washing under the hood safe for your engine and other parts? It would be nice to have the engine look really clean...but I obviously wouldn't want to damage any parts in the process. Is washing the engine at a self service car wash safe?