What you're describing is a Project ENGINEER's job, the the Project MANAGER.
As the Manager, it's your job to make the client's intended outcome happen. You do this by having a clear understanding up-front of what they want, and then continually monitoring the progress against what's expected. In doing this you might find even ways to improve something, or a way to correct something.
One earlier question here was from a PM asking how he should handle a a situation where he found a problem that, while not affecting his project, could affect the product of the project several years down the line. That's adding value.
You've been entrusted with money, time, resources and authority to bring to fruition something they think will benefit them in some way. So focus on being responsible with what they've given you, and trying to make sure that the project delivers as hoped.