2002 496 MAG-HO(185hrs fresh water all it's life)) with closed cooling and according to other's the PSI is too high. Nearly 40psi around 4000rpm according to rinda live data. So since I don't have much service info other than the former owner replaced the impeller in 2018... where would be the likely places for the vanes to get stuck, if there once was a failure causing the issue and where to pull the hoses to check? Engine runs fine and about 160*F?

thanks

40 psi should be fine with closed cooling.

40 is a little high at 4000rpm(depending on mph and water pickup) IMO.

I would take the hose off the rear of the oil cooler and see if anything is blocking the tubes. That's where it always happened to me. I would know something was blocking the flow looking at the water pressure gauge.

496 max is 30 psi per OEM spec

I always take the right side cap (looking at the engine ) off of heat exchanger and flush, remove bigger hose on right side of changer and remove right side hose off of cooler that is in back of motor about intake height, flush from front to back and watch to see if parts of the impeller come out, There is a cooler on the lower right side that has a screen to stop debris form passing any father. Best way I can explain it.

I presume will need new gasket/seal for the cover? Will be getting boat back on land this week to check closer...

Yes, plus they sometimes suck that gasket into the heat exchanger.