I am having trouble with over heating. Originally, I thought I'd sucked up seaweed and or other debris, I removed the impeller, it was older but no missing fins. I replaced the impeller, but still no water coming out thru hull exhaust.
I flushed the raw water system, I took apart 90% of the hoses and joints and flushed, didn't find anything significant.
I noticed a score in the seal at the end of the impeller, so I ordered a reman entire pump.
now, I finally have great flow, and plenty of water coming out the exhaust, but still over heating.
I replaced the tstat, still over heating.
Is there a way to tell if the water pump on the engine coolant side being bad?
replacing the engine water pump looks to be a much larger project than I want to attempt, however we really don't have many boat mechanics in our area, and so before i commit to attempting, I am hoping someone might have alternate ideas?
ps, 1 manifold was nice and cool, but the other was pretty warm, but having flushed everything, I can't imagine anything being the problem in the raw water side?
thanks in advance! Hopefully someone with much greater knowledge than i will see this!!?

Check the oil cooler sea water inlet on the port side under the exhaust manifold of you have good flow out of the other side. That was the first thing that would got clogged on mine.

Could use a motor serial number to see how the motor is configured

Wonder if it's your gauge
Have you used a IR temp gun to verify it's actually over heating?

Is it overheating or just running hotter than normal? Is it "overheating" at idle, cruise, wot? Have you ever changed your antifreeze? Have you ever changed your heat exchanger end gaskets? You'd be surprised how much flow they can reduce when the get sucked in

Not sure sure if this helpful but I just installed two new to me 496HOs into an old cruiser. I found on ebay VesselView Mobile devices for less $100 each) ...they are bluetooth dongle you plug into your 496 harness/CPU and you get great real-time data on your phone.

for me, seeing Fuel, Oil, and Water PSI, Water Temp, charge volts, Fuel Consumption, and accurate RPMs is really useful. I actually purchased two used Iphones and throw them on the dash when doing longer/faster runs - one per motor. Was shocked how inaccurate gauges were.

Did you ever figure out the issue?

BTW - was thinking it would be cool to have the data push to one of my two garmin screens ... couldnt figure out what was needed so emailed Mercury. Nice detailed response and when I priced all the cables, dodabs, and dongles things it was upwards of $800 to see data I can already see. Criminal

Fox Marine and Yacht Devices make NMEA 2000 gateways for Mercruiser engines that are way cheaper than going with the Mercury one.
https://fox-marine.com/index.php?rou...&product_id=68
https://www.yachtd.com/products/engine_gateway.html

There are lots of posts on this over the years.
Antifreeze is only good in these motors for 7 years, tops. As soon as it starts to turn dark, it needs to be drained and replaced. When it's dark, it is silting up.
Definitely need to open and clean the heat exchanger, and replace the end rubber gasket caps as a maintenance item. The pic above indicates how they create blockages.
How old is the seawater pump impeller? Is it flowing even water out of both exhaust tips? Your SWP impeller could be shot. Your SWP could also have wear not allowing enough water pressure for cooling.

Once you have done all of the above, report back.