A C414AW IN CRUISE AT FL270 STARTS A CLB AND EXPERIENCES A FIRE WARNING IN #1 ENG. PLT DECLARES AN EMER AND DIVERTS TO PDT.

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WHILE CRUISING AT FL270 IN IMC. I REQUESTED AND WAS GRANTED CLRNC FOR HIGHER ALT TO REACH VMC. THE ACFT WAS CONFIGURED TO CLB PWR USING XYZ COMPANY PWR SETTING. DURING THE CLB; THE L ENG FIRE LIGHT ILLUMINATED. THERE WAS NO VISIBLE SIGNS OF FIRE FROM THE ENG; BUT THE FIRE LIGHT DEMANDED FOLLOWING THE PRESCRIBED EMER PROCS UP TO AND INCLUDING DISCHARGING THE FIRE EXTINGUISHER. SINCE THE LIGHT WAS STILL ILLUMINATED; THE ENG WAS SECURED. ZSE WAS ADVISED OF THE FIRE LIGHT; ENG BEING SECURED 'AS A PRECAUTION' AND A 'PRECAUTIONARY' LNDG AT THE NEAREST ARPT WITH A 5000 FT RWY WAS REQUESTED. ATC VECTORED TO PDT; OR; ABOUT 25 NM AWAY. THE ACFT BECAME VMC AT ABOUT 7000 FT MSL AND 10 NM W OF PDT. A SAFE SINGLE ENG LNDG WAS MADE. THE PLT WAS REQUESTED BY AND PROVIDED INFO (PERSONAL; NAME; PHONE; CERTIFICATE NUMBER; ETC) TO TWR FOR RPT CTR WAS MAKING. A VISUAL INSPECTION WAS MADE BY AN A&P STATIONED ON FIELD AND PLT WHO IS ALSO AN A&P. NO SIGNS OF FIRE OR HEAT DAMAGE WAS FOUND WITHIN ENG COMPARTMENT. THE FALSE FIRE INDICATION IS BELIEVED TO BE CAUSED BY INSUFFICIENT COOLING THROUGH MIXTURE CTL; AND SINCE ENG IS LIQUID COOLED; THERE IS NO CIRCULATING AIR AROUND TURBO/SPOT DETECTOR. (NO COWL OPENINGS FOR ENG COOLING.) THE FIRE DETECTOR HAD TO WAIT FOR THE ENG TO BE SECURED; AND TEMP AROUND/WITHIN THE TURBO/SPOT DETECTOR HEAT SHIELD TO COOL EXTINGUISHING LIGHT. ENG AND COMPARTMENT FURTHER EXAMINED AT DEST AND SVC REQUESTED FOR FIRE DETECTION SYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THIS ACFT IS PWRED WITH CONTINENTAL LIQUID COOLED ENGS EQUIPPED WITH TURBOCHARGERS MANUFACTURED BY THE RAM CORPORATION. THE RPTR SAID THE TURBOCHARGERS RUN EXTREMELY HOT AND ARE COVERED WITH A HEAT SHIELD BOX AND THE FIRE WARNING SENSOR IS LOCATED INSIDE THE BOX. THE RPTR SAID SINCE THE ENG IS LIQUID COOLED VERY LITTLE COOLING AIR FLOWS AROUND THE HEAT SHIELD AND THERE ARE NO VENTS OR HOLES IN THE COWLING. THE RPTR SAID THAT INADEQUATE COOLING OF THE TURBOCHARGER HEAT SHIELD MOST LIKELY CAUSED THE FALSE WARNING. THE RPTR STATED THE COOLING IN CLB MUST BE CTLED BY THE FUEL MIXTURE CTL PER THE MANUFACTURER'S DATA.