Use an IR temp gun on the crossover housing to verify its not over heating. If its not then probably need to replace the engine temp sensor (805218T)
which is behind the water pump pulley
Item 7
Genuine Mercury & Mercruiser parts. CLOSED COOLING SYSTEM

Do you have a Rinda scanner or similar?
You can't troubleshoot these motors without a scanner.

What is your water pressure?
Are you seeing equal water dump from each exhaust?
Are both sides of the exhaust the same temperature (with a laser temp gun)?
How old is your antifreeze? Is it bright pink, or darker, more metallic looking? The AF is only good for 6-7 seasons, tops, then it needs to be drained and refilled with fresh; as it silts up, does not flow or cool as well.

year model and engine serial is a MUST when help is needed and or working on marine engines. Next is your apps totally 100 percent OEM stock and set up ?

Stock OEM -- PCM 555 apps ---- There is 4 different alarms - ie Beep then goes away. Beep / pause / Beep and so forth for alarms. Without knowing which beep / warning horn your are experiencing one could not assume 100 percent its the overheat alarm, that would be a guess.

All Marine EFI / MPI engines need be scanned for engine diagnostics / problem solving , It is even best to scan before one gets after it for the start of boating season and during as well to help detect possible issue(s) before a real issue . Also engine scanning is needed especially when issues arise including warning horns going off presently..

If you are close to Texas, I know this would not be hard to fix nor time consuming by no means, unless you have other issues lurking.

Has the engine been scanned to verify which fault is being thrown? If not, it needs done before proceeding any farther.

Thank you all for the info and advice. My gut feeling thinks that there may be some pieces of impeller still stuck in the system. When I took the caps off the heat exchanger there were some small pieces in there from an impeller that went out on me last trip out. I put a new impeller in but did not back flush the system. Can someone explain to me how to perform this procedure? It is a bone stock 496 mag ho
thanks

While back flushing is running water in reverse to the pump. If (the big IF) your actually overheating then you need to know that. In post 1 you've indicated the gauge does not show it. Get a cheap temp gun at Lowes, Depot or online and see if it is or is not


If it is overheating, then pull hoses and flush water in reverse, open end caps and flush

Don't fool yourself - just running water through a cooler that is clogged generally won't help you much. You need to put eyes on the cooler (with your eyes or cell camera) and determine if there is a restriction. When I've had a restriction, it took a straightened coat hanger or small welding rod to push the impeller pieces, pebbles, shells, etc out of the capillary tubes. Water just won't cut it, generally speaking.

Good luck.

what is the normal operating temperature for this motor?