Your privacy in a vehicle is protected only insofar as something you might be carrying in a locked storage compartment (glove box, trunk, or in some cases armrest). If the storage compartment is locked and a police officer wants to open it they are only legally allowed to do so with a warrant. Now if the only way to access your engine requires you to open a locked storage compartment like on certain mid-engined cars, then the cops would need to get a warrant to look at your trunk to look at your engine. But otherwise you've pretty much gotta let them see.