I have a 454 mpi running at 175° cruising speed and drops down to 160 at Idile is this normal? I just replaced the impeller and thermostat I put a 160° thermostat in it

That is right where my 502 runs at. I would call it normal.

Normal.

+1 Mines running in 100*+ air temp and 90* water. If I'm running hard I usually set a high idle on the "final approach" and she jumps back to 160* pretty quick.

Sounds normal.

OK thank you I think I was just paranoid ha thank you for the responses!!!

The black plastic Impeller housing and wear plate and O ring gaskets should have been replace with the new impeller or did you do this as well ?

Double check backside of oil cooler for blockage of whatever including old impeller pieces that could cause a slight water flow restriction

Next how old is your water circ pump on the front of the motor - get an inspection mirror out and check at the weep hole for rust stains and or any water leaks at the weep hole - if so it is time to change your water circ pump. I am not saying your engine is overheating but I think you could drop that 175 degree down a tad at full throttle to closer match your idle speed temps.

It is always best to try to keep your temps steady for a fully warmed engine. Just some double checks here to see if you can close that 10 % temp gap if all possible.

Most of the time when we see larger temp gaps it usually means its time to change the water circ pump on the front side of the engine if applicable. Depending on what you have and the waters you boat in plus how well you winterize properly or do not can depend on the condition of the water circ pump and longevity . .

Yes I replaced the metal plate and the ordering everything that came in the kit along with the plastic housing I boat in fresh water and no leaks from the front water pump but I will look into it more

And the "o ring" I ment to say damn phone auto correct ha