I have a 270 1998 searay with twin 4.3 efi engines. The port engine overheated last year and I replaced the impeller which corrected the problem until last week. I have pulled the boat for the winter but this time replaced the engine thermostat, hoping to correct the problem. Old thermostat open when placed on my stove and heated water. Engined only have 100 hours and the impeller in the outdrive didn't look bad when that was changed. Anyone have an idea if there could be something else causing the overheating. Engine temp gauge stops at 170 when ideling but goes up when the rpm's are increased.