Open-ended problem solving is a skill that is central to engineering practice. As a consequence, developing skills in solving such problems is imperative for engineering graduates. Open-ended problems are often ill-defined and can have more than one viable solution. This paper presents preliminary results from a mixed methods study examining the specific problem solving strategies of undergraduate engineering students when dealing with the ambiguity of open-ended problems. Participants completed measures of content knowledge, epistemic beliefs, and working memory, and also solved four complex engineering problems during individual problem solving sessions. A subset of participants solved these problems during a think aloud session and participated in follow-up interviews. Preliminary analysis of quantitative data has focused on identifying correlations between problem solving solution scores and the measures of epistemic beliefs and working memory capacity. Analysis of qualitative data is examining the problem-solving processes students.