I am new to inboards. Someone told me that you should change the impeller on your motor every so often.I have a 454 big block Mercruser inboard 1999.I am pretty good with tools and have changed water pumps on a car. went to the dealer and they sold me an impeller. and a o ring. I am looking and can not figure out where this goes.It has been around 250 hours but the motor is still running around 175 degress the same as i bought it should I worry and is this the right impeller.the motor is a fuel injected 7.4.