Where is the ecm located on a 91 Buick century 3.3?

On a 1991 Buick Century with a 3.3L engine, the Engine Control Module (ECM) is typically located under the hood, near the passenger side inner fender well. It's often mounted to the firewall or a nearby support bracket. It's a fairly large, rectangular box with many connectors. However, the exact location can vary slightly depending on options and specific manufacturing details. You might need to do some careful searching in that general area.