My '02 275 with 496 HO has been running on plane and idling at 180 degrees. I can't remember how hot it was running last year. Does the 496 have 160 degree thermostat? Is 180 a potential problem? What temps are others 496's running? Also, I've heard that 496's have a brass impeller, is that true?
Any input would be greatly appreciated.

My Merc. 496 service manual list the thermostat at 160.

I believe the motor goes into safe mode at about 190 degrees.

Have you changed the impeller for the raw water pump (neoprene?)?

Mine runs about 165-170. It only gets to maybe 180 after a hard WOT run, but cools back down quickly.
Time to check impeller.

Thanks for your input guys. I'm going to attempt to pull the seawater pump and replace the impeller. Do I need to lubricate the new impeller? Anything else I shouold know?