Abstarct: This qualitative multiple case study investigates the effectiveness of diagnostic assessment in improving EFL learners' comprehension and production of English collocations and idiomatic exxpressions. Two intermediate-level Iraqi EFL learners, Sarah, a strong student, and Taha, a weaker student, participated in a diagnostic-coaching intervention designed to identify and address their specific collocational and idiomatic gaps. The study employed a multiple case study design, utilizing semi-structured diagnostic interviews, a targeted one-on-one coaching session, one-week Collocation Logs, and exit interviews with reflection questions to document learners' progress. Data analysis involved comparative transcxript analysis to identify missed opportunities, incorrect usage, L1 transfer, and evidence of "up-cycling", the qualitative shift from literal to idiomatic phrasing. Results revealed distinct patterns of gaps for each participant. Sarah demonstrated missed opportunities to use natural collocations and idioms (e.g., "make a talk" instead of "have a talk"), while Taha showed more fundamental challenges including literal interpretation of idioms (e.g., interpreting "break the ice" as physically breaking ice) and basic collocational errors (e.g., "strong rain" instead of "heavy rain"). Following the one coaching session and one-week logging period, both participants demonstrated meaningful up-cycling. Sarah shifted from "I decided to talk to him about the problem" to "I decided to have a word with him about the issue," and from "I was very tired" to "I was completely burnt out." Taha progressed from literal interpretation to figurative comprehension of idioms and corrected basic collocational errors. Reflection statements revealed increased confidence, mextalinguistic awareness, and developing learner autonomy. The findings demonstrate that diagnostic assessment, combined with targeted coaching and personalized feedback, effectively identifies specific learner difficulties and facilitates measurable improvements in multi-word exxpression comprehension, even within a brief intervention period of one coaching session followed by a one-week logging period. This study contributes to the underexplored domain of collocation and idiom diagnosis, offering pedagogical implications for integrating diagnostic practices into EFL vocabulary instruction. چکیده