IMHO a "project manager" is a person who does the least work on a project, but gets the credit when it is successful. Conversely, when it isn't successful, they have the power to delegate responsibility for failure to someone else.
Very often a "project manager" on a project is also a "project manager" on one or more other projects simultaneously, which means of course he is not a full-time manager on any one project. So the project requires a "lead engineer". This "lead engineer" is the one who will get the flak when things turn bad, but not the credit when things go well.
A real "project manager" will do all he can to make the job of the "lead engineer" difficult, and of course in doing so, he will make the "lead engineer" totally responsible for the difficulties, often quoting some unwritten "law" that says "lead engineers" should anticipate the changes and new deadlines that "project manager" can introduce at any time.