What was found
INFLIGHT REPORTED BURNING PLASTIC SMELL & HOT AIR BLOWING OUT OF VENTS IN AFT CABIN. SMELL MOVED FORWARD TO COCKPIT. NO SMOKE, TURNED OFF RECIRC FANS. SMELL CONTINUED, RECEIVED PACK NR 2 FAIL EICAS. PERFORMED QRH. DID NOT TURN PACK NR 2 BACK ON, SMELL DISSIPATED. DECLARED EMERGENCY. FOUND PACK NR 2 BYPASS VALVE FILTER CONTAMINATED. REPLACED BYPASS VALVE. FOUND NR 2 ACM CONTAMINATED. CLEANED & INSTALLED BACK ACM. FOUND RT & LT RECIRCULATION FILTERS CONTAMINATED DUE TO INSULATION BLANKET INGESTION. REPLACED RECIRCULATION FILTERS. INSTALLED INSULATION BLANKET AROUND PACK 1 & 2 DUCTS AND RECIRCULATION FILTERS. SECURED ALL DUCTS BACK TOGETHER. BOTH PACK SYSTEM OPS CHECKED OK WITH ENGINE RUNNING.