MOP

A few things I have picked up about the pressure issue, most head gaskets start to weep "externally" around 30psi. Quite a few feel 20psi should be the max in the system. The Merc valves are set for 30psi that leaves you on the edge. Dumping water into the exhaust increases back pressure hurts speed it should go over board, no matter who's valves you use it must have a strainer ahead of it or it will be useless in no time!. Want to check out a high quality adjustable valve check out http://straval.com/ I feel they are better quality and also less expensive then the Merc valve.

Phil

BenPerfected

You do have the thermo housing with the hose dumps to the exhaust that by-pass the thermostat. I don't get why you have high WP at low RPM's/speed if you are measuring pressure at the intake with the hoses to the exhaust manifolds disconnected. If all the above is true, try an small rubber hose in an 1/8" NPT hose fitting in your intake to a mechanical WP gauge to check WP. You can also look for a restriction in one of your hoses. This will make sure you aren't chasing ghost.
For over 1 yr, we looked for an overheating problem in one of my Mercury engines. It turned our to be a defect from the factory in the Merc thermo housing (similar to yours). The restriction was from a 1/2" NPT red plastic thread protector that got pushed in vs removed when the foactory NPT fittings were installed. After a few yrs of bouncing around inside, it finally wedged in a passage, and we could not see it. After we finally pinned down down the is issue/location and replaced the housing. We then cut the old housing in half with a band saw and found the POS plastic...f_ _ _ me.
I am now drinking a beer to celerbrate this 15 yr old discovery...again! I understand your frustration.