What was found
ENROUTE FROM KCVG TO KBDL CREW REPORTED FWD EQPT OVHT EICAS MESSAGE. CREW COMPLIED WITH ATI QRH; CABIN STARTED TO CLIMB. DIVERTED TO KBUF & AN EMERG DESCENT. LANDED WITHOUT INCIDENT. ACFT GROUNDED. DURING FAULT ISOLATION, LINE MX REPORTED WITH LT ENG ENGAED PER AMM 71-00-00 & OPS OF 1 PACK AT A TIME, PRESSURIZING ACFT IAW AMM 05-51-24, LT PACK UNABLE TO MAKE 1 PSI DIFF PRESSURE. RT PACK MADE DIFF PRESSURE. INSP OF LT ECS BAY SHOWED ACM INTAKE PLENUM COUPLING WAS NOT SECURED. IAW AMM 21-51-03. TROUBLESHOOTING IDENTIFIED LT PACK MAKING INSUFFICIENT AIR. LT PACK R & R IAW AMM 21-51-03. OPS & LEAK CHKD GOOD. NO FWD EQUIP COOLING OR OVERHEAT EICAS MESSAGES & LIGHTS DISPLAYED THROUGH THE COURSE OF FUNCTIONAL CHECKS.