AN MU2 MARQ ON INITIAL CLB AT 2000 FT DECLARED AN EMER AND DIVERTED DUE TO SMOKE IN THE CABIN CAUSED BY A SLOW OPERATING HEAT CTL VALVE OVERHEATING FIBERGLAS DUCTING.

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THE PROB OCCURRED DUE TO THE ENVIRONMENTAL SYS. THE MOTOR THAT TURNS THE VALVE TO DUCT HEAT IN WAS OPEN MORE THAN SUSPECTED; BUT DUE TO THE PWR BEING AT FLT IDLE WAS NOT A PROB. AT TKOF WHEN THE PWR WAS SET FULL; THE TEMP INCREASED TO SUCH A TEMP THAT IT BEGAN TO MELT AND OVERHEAT THE DUCT WORK OF THE PLANE. AFTER TKOF AND INTO THE CLBOUT THE SMOKE FILLED THE CABIN. WHILE TRYING TO DETERMINE THE PROB; I INFORMED APCH OF THE SIT AND BEGAN TO VACATE THE CABIN OF SMOKE. THE CTLR VECTORED ME AROUND FOR AN APCH (ILS RWY 26R) AFTER LNDG AND MAINTAINING CTL OF THE SIT I RETURNED TO FBO; EXITED THE CABIN THROUGH THE FRONT WINDOW AND WALKED AROUND TO OPEN THE REAR CABIN DOOR TO DETERMINE IF THERE WAS ANY FIRE OF WHICH THERE WAS NOT. AFTER INVESTIGATING THE PROB AND DETERMINED THAT THE MOTOR TURNING THE VALVE WAS SLOWER THAN NORMAL I GAVE IT MORE TIME TO CLOSE AND CONTINUED THE FLT WITHOUT A PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A MITSUBISHI MU2 MARQUISE PWRED WITH AIRESERCH TPE 331-5-252M TURBOPROP ENGS. THE RPTR STATED THE ACFT HAS AN AIR CYCLE ENVIRONMENTAL SYS USING ENG BLEED AIR AND WAS NOT AWARE OF THE SLOW MOVEMENT OF THE HOT AIR VALVE. THE RPTR STATED THE VALVE WAS NOT REPLACED AS THIS WAS CONSIDERED NORMAL OP.