TWIN ENG SMT ACFT ENG GND COVER CAUSES ENG SHUTDOWN AFTER DEP WITH LOAD OF PAX.

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ACFT WAS AN SMT ACFT. PREFLT DUTIES GO TO FO. NO MENTION WAS MADE OF AN ABNORMALITIES EXCEPT FOR RECENT MAIN ON #2 INVERTER. WHENEVER I APCH ACFT FOR FLT; I CHK THAT ENG PLUGS ARE REMOVED; PROPS UNSTRAPPED; TAILSTAND STOWED AND CHOCKS PULLED. ALL WAS NORMAL. ENG START; TAXI; RUNUP; AND SHUTDOWN AT GATE NORMAL. WE WERE LOADED AND RELEASED; BRIEFED AND ENG STARTUP; TAXI AND TKOF ALL NORMAL PER REGS; COMPANY PROCS; AND MANUFACTURER'S CHKLISTS. 10 MINS INTO FLT; I NOTICED OIL TEMP ON L ENG (TURBOPROP ENG) WAS AT REDLINE. I REDUCED PWR AND INFORMED FO. I ASKED HIM TO TAP GAUGE TO SEE IF STUCK AND TO MONITOR TEMP. ALL OTHER ENG INSTS NORMAL. TEMP CONTINUED TO RISE; REDUCED PWR FURTHER AND CALLED MAINT ON RADIOS. MAINT AGREED WITH MY DECISION TO SECURE ENG. ENG SHUTDOWN; RETURN TO ANC AND LNDG WERE NORMAL. UPON OPENING OF ENG COWL BY MAINT; WE FOUND AN ENGINE PLUG WEDGED IN OIL COOLER. WHEN I WENT TO STOW IT; I FOUND 2 MORE PLUGS. I ASKED FO WHERE EXTRA PLUGS CAME FROM. HE SAID WE WERE MISSING 1 DURING PREFLT SO HE OBTAINED 1 FROM MAINT BUT DID NOT THINK IT WAS IMPORTANT ENOUGH TO MENTION. HUMAN PERFORMANCE: I AM TOO TRUSTING OF OTHERS TO PERFORM THEIR JOBS ADEQUATELY. FO HAS BEEN PASSED OVER REPEATEDLY FOR UPGRADE. POSSIBLY BORED OR ANGRY. I WAS UPGRADED RECENTLY OVER SEVERAL OTHER PLTS. ENG PLUGS SHOULD BE SECURED TO PROPS TO PREVENT INGESTION BY ENG.