As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN FLIGHT DURING INITIAL CLIMB OUT, THERE WAS A CABIN ODOR PRESENT THAT SMELLED OF BURNING RUBBER. ODOR WAS STRONG & PERSISTED THROUGHOUT FLIGHT & THE RETURN TO AIRPORT. ODOR SEEMED STRONGER IN CABIN. REPLACED SEPARATED PRECOOLER WITH NEW UNIT IAW MM 34-11-05. OPS AND LEAK CHECKS ARE GOOD. NO ODORS ARE NOTED DURING ENGINE RUNS WITH BLEEDS ON.