This article aims to describe and analyze the dilemmas facing teachers when fostering student motivation in project-based learning (PjBL). The authors developed a project in secondary education through an action research approach. The different action research cycles aimed to solve dilemmas throughout the project and interpret them in terms of student motivation through the self-determination theory. According to this theory, autonomy, relatedness and competence are three psychological needs that, if fulfilled in the classroom, will lead to greater intrinsic motivation. The results revealed that one of the main characteristics of PjBL, namely, presenting the final product to an external audience, led to a tight schedule. This time-pressure context generated the majority of dilemmas, which affected the development of the three basic psychological needs, especially competence and autonomy.