What was found
INSPECTION TYPE:SI, AIR TURNBACK AT 1945UTC DUE TO UNSATISFACTORY AIRCRAFT PRESSURIZATION. WHEN CLIMB THRUST SET, CARGO HEAT O'FLOW VALVE CLOSED, FULL MANUAL INCRASE ON MANUAL CONTROLLER. AIRCRAFT WAS ONLY ABLE TO MAINTAIN A CABIN ALTITUDE OF 8,000 AND DIFFERENTIAL OF 7 PSID. HAD FLIGHT CONTINUED, CABIN ALTITUDE WOULD HAVE EXCEEDED 10,000 2-3MTS. TROUBLESHOT SYSTEM PER MM, FOUND RT OUTFLOW VALVE TO BE STUCK OPEN DUE TO INSULATION BLANKET STUCK IN VALVE. REMOVED BLANKET AND PERFORMED PRESSURE CONTROL SYSTEM TEST PER MM 21-30-01. NO DEFECTS NOTED.