As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) AFTER TAKEOFF THE FLIGHT CREW NOTICED FUMES IN THE CABIN, AND QUICKLY DETERMINED THE PRESENCE OF SMOKE. THEY DECIDED TO DECLARE AN EMERGENCY AND RETURN TO THE AIRPORT. THE AIRCRAFT LANDED WITHOUT ISSUE. WHILE INVESTIGATING THE CAUSE IT WAS FOUND THAT A CHECK VALVE IN THE FUEL PURGE SYSTEM WAS FAULTY, AND IT ALLOWED FUEL TO BACK UP INTO THE FLOW PACK OF THE RELATED ENGINE. ONCE FUEL ENTERED THE FLOW PACK, THE BLEED AIR SYSTEM CARRIED THE FUMES/SMOKE TO THE CABIN AIR VENTS.