You went and put a closed cooling system on an engine that was set up with a computer for a raw water cooled engine did you change computer to the one for closed cooling they are not the same at all from timing to a lot of other stuff. If youII
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Why would you take out a 454 and replace it with a smaller engine?
I guess a better question is why wouldn't you replace it with the exact same engine? From what you are saying it sounds like the engine is not up to the task. Needs less pitch in the prop and needs to spin 400 rpm more to get max power. Obviously works but is stressed alot more for a little more power. Why not the other way like a 496? I ask all this because of what little experience I have in boats. When I was shopping for a boat I tested a sundancer 260 and it had a 350 MPI it needed to be at about 3700-4000 to be doing about 25 knots. The mechanic in me didn't like running it constantly at such a high power setting. When I test drove the 280 that I bought I was happy to see the 4.3s spin about 3200 rpm to get the same speed. I forget exactly how much more fuel the 280 burned but I know that it was a little bit more. Not sure if it was because of being a little bit heavier of two less efficient alpha drives hanging in the water. Those with more experience than me I would love to hear from you!
If your cooling system wasn't doing it's job, the engine would overheat. Alarm would sound, and it would be recorded on the ecm. More likely, it's a fuel delivery problem or lack of it, which the ecm wouldn't record.. Hole in pistons, damaged spark plugs, are a result of sky high cylinder temperatures and could happen without an increase in coolant temp.
Look that over real close or you'll be cooking another one.
I think the engine was never right out of factory. A 6.2 mpi is direct replacement for the 454 and should spin about the same pitch prop. If you had to go from a 24 to an 18 pitch prop then it lacked massivly in power before finally failing. A lean burn might indeed be the answer.
Just got boat back in water with new 6.2. Ran at 5,150 RPM at WOT with 18 pitch Hill Marine 4 blade cupped wheels. WOT is supposed to be between 4,800 and 5,200. Impressive hole shots. For now, I am very pleased. (I did not re-gear outdrive). Mercury is warrantying this new engine but only if don't re-install the Monitor 5320 cooling system that I paid $1,200 for. I didn't install it and it is for sale. Mercury claims the aftermarket cooling system caused the valves to tulip and plug electrodes to burn up. Monitor says this is unheard of and the original engine installer said they have installed many many Monitor systems and never had this issue.