Learning to use critical practice-based research as part of teaching is an important goal for preservice teachers, especially for those who plan to teach English learners in linguistically diverse settings. In this study, we examine the experiences of preservice teachers who were introduced to a framework for enacting iterative, transformative action research, and used the framework to study their own teaching in a one-on-one writing partnership with young English learners. Using an established self-efficacy survey instrument, as well as qualitative measures such as course artifacts and observations of teaching, we conducted a mixed-methods study to examine the impact of research engagement on preservice teachers' self-efficacy, self-reported knowledge of practice-based research, and agency. Findings suggest that the experience helped preservice teachers grow in their knowledge of practice-based research and reflect on their teaching decisions, but gains in self-efficacy varied across participants in relation to their racial and linguistic positionalities, their understandings of race and language, and their successes and challenges with enacting critically-oriented research. This study has important implications for the design of preservice teacher education that emphasizes the role of research in teaching and supports the preparation of teachers for English learners in linguistically diverse communities.