thanks for the info when your friend had the high water pressure problem how high was it....

If you thread your gauge into the intake cts port or anywhere that reads whats in the block and hit a genuine 40 thats freeze plug poppin intake gasket seal compromising criculating pump seeping kind of pressure there. Not always, but likely . ,
see what cmi requires for rough gpm flow per header to keep them healthy your engine or exchanger may be generating more heated water that the cmis need or want

Maybe I'm missing something but the raw water pressure doesn't apply to the freeze plugs, etc. as noted. Certainly the various heat exchangers experience that pressure but the block is part of the closed system which is regulated by the cap on the overflow tank. BUP jump in here

(following comments are for water/cooling references only) Just looked at the Merc diagnostics manual and the only alarms or Guardian Activated situations occur for Low Seawater Pressure BLK PSI LO which will sound alarm and go to Guardian mode, coolant over temp ECT OVRHT also, block pressure sensor failure will give beeps but not go Guardian and coolant temp sensor failure will give beeps but no Guardian. I don't see any capability for the computer to indicate excessive raw water pressure and go to Guardian.

The block pressure is what would wake me up at night . If you have 50 at the pump outlet and its been good for years hey roll with it . If you have a exit problem ect and have 50 psi in the block its not all good .
if reads 40 wot at sensor that tells you if the pump is pumping 20 in the block 15 at inlet to headers and its not running hot then yea i wouldnt get freaked out about it

Doh ! Heat exchanger ! Prob big bad boy too . Removeable end caps i hope.
Now we are back to the guy at cmi ,and a poppet valve dump. Might get an extra 30hp you didnt know you had

The 50 psi is in the raw water side (heat exchangers, headers, not in the block)

I think this is the best route to take to try to solve this. It's not very likely that there is a restriction in the headers but you can try blowing them out with compressed air at about 70 psi. Good chance the heat exchanger is the culprit for an obstruction.

Another boating season.....Still have my high water pressure problem. My 525 is in the shop having valve springs and head gaskets. I now have 200hrs on the engine. Pulled the headers today and I'm off to CMI in the morning, David Rank at CMI seems to be very interested in my high water problem. We will be testing the headers and tail pipes for flow and pressure. I will post our results. Thanks for everyone's interest

i still think you have small peices of rubber& some other junk stuck in one of the coolers.as already stated,they need to be taken off the engine and rodded out.i had one in the shop a few months ago ,it had been backflushed several times before it came to me.i could not believe how clogged the cooler was.