GA SMA IN DIVERSION TO ALTERNATE WHEN AN ACFT EQUIP PROB MALFUNCTION IS EVIDENT.

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ON AN IFR FLT FROM 5D8 (MARSHALL; MI) TO CHICAGO MDW; I NOTICED AN ABNORMALLY HIGH OIL TEMP AND DECREASED OIL PRESSURE. I ADVISED SBN APCH THAT WE WOULD LIKE TO LAND AT SBN INSTEAD OF PROCEEDING TO MDW (WHICH WAS LOW IFR AT THE TIME). THE CTLR CLRED US DIRECT THE VOR AND THEN THE FIELD. LATER; HE ASKED IF THERE WAS A PROB. I ADVISED THEM THAT WE HAD LOW OIL PRESSURE AND HIGH OIL TEMP; AND WERE MERELY MAKING A PRECAUTIONARY LNDG. WE WERE SUBSEQUENTLY CLRED FOR THE VISUAL AND PROCEEDED FOR RWY 18. HE THEN ADVISED THAT 'THE TRUCKS' WERE STANDING BY. I REPLIED THAT THE TRUCKS WERE NOT NECESSARY AND THAT EVERYTHING WAS WELL AT THE TIME. APCH STATED THAT THEY HAD CALLED OUT THE TRUCKS AS A PRECAUTION EVEN THOUGH WE HAD NOT INITIATED A 'PRIORITY' OR AN 'EMER.' WE LANDED WITHOUT INCIDENT. UPON INSPECTION BY AN A&P; IT WAS DISCOVERED THAT THE OIL COOLER WINTERIZATION PLATE HAD NOT BEEN REMOVED. IT WAS NOT VISIBLE DURING THE PREFLT AND THEREFORE I DIDN'T CATCH IT. PERHAPS AN INSPECTION OF THE ACFT LOGBOOKS WOULD HAVE REVEALED THIS. THIS HAD CAUSED THE ENG TO OVERHEAT. A MORE CAREFUL EXAMINATION OF RENTAL ACFT WOULD BE BENEFICIAL IN THE FUTURE.