This is a perfect example of when you have to the rear hoses. In this system there is no way for the water to flow when the thermostat is closed. There is no circulation pump or crossover tube from the suction side to the pressure side of system. The hose from the inlet crossover to the thermostat housing is designed to let the incoming water go strait out the headers while the thermostat is closed. If you have this type of system by all means run the hoses so the engine can be cooled or just run a restrictor plate instead of a thermostat. You'll have to test different sizes. No thermostat without some restriction is not an option.

If you have a stock mercury system the thermostat will do all that for you. Dumping water off the back on this system will cause the water in the engine to run cold closing the thermostat increasing the water pressure and possibly overheat with little or no warning. When you start burning valves head gaskets or pistons your temp gauge still be reading normal and then someone will tell you bought bad gas.

Thanks for posting your pictures

I forgot to mention that I have 3 holes in my thermostats so I do have water flow. I did this to propagate flow through the front of the engine. I also take my water pressure and temperature at the front of the engine under my thermostat. In the pic the engine on the left you can see my temp sender. The engine on the right you can see where I drilled and tapped the intake for my water pressure gauge.
I have plugged the bypass hose on the crossover and it only increased my pressure 6lbs, but my exhaust got a lot louder and hotter. (dry to the tips)

The three holes in the thermostat are not enough with a raw water system this why the hoses fixed your system. A restricter plate would have done the same. The result you found by removing the crossover is that your thermostat has enough cold water around it to close, so the three holes were the only water exit, increasing water pressure and starving the exhaust.

Your valves probably stuck because guides weren't expanded enough to lubricate the stems. The valves are more likely to tulip if seat is too hot. The thermostat controls the circulation of coolant inside the engine and lets a little amount in from the radiator(lake) until the differential allow a preset temp to be reach. A circulation water pump is need to accomplish this task among other things. This is impossible with the raw water system the lake or seawater is only 60 to 80 degrees F and will close and open thermostat eradically.

The placement of your water temp and pressure is personal preference. The gauges will only warn of a properly operating system when it has started to fail. Knowing how it suppose to work is up to you and/or the designer. As I said before you found the right update for your system. The original post didn't state what type of system was to be modified tried to give examples of the common ones and the pros and cons. You know if aint broke. My boat shows 35psi any speed over 120mph I know this is much higher than needed,but I have never had anything fail because of it. If I were to change it who knows. Yes it would be faster and my research has showed there shouldn't be any adverse effects. I mean really how long can you run 180mph and who really cares. The gauge still goes to zero if the no water pressure and they read the same amount if the boat trimmed right that good enough for me.

Just out of curiosity could it be possible to instead of drilling/tapping the rear of the intake manifold to replace the plugs on the rear of the heads with bushings/nipples and then run a PRV to dump overboard or thru the exhaust to relieve the hot spots and pressure?

Sure, you might find their pretty hard to get out. You can dump the water from anywhere and it will have the same effect. To much and burn the headers, exhaust to pieces all the water being pump through the engine is to cool the exhaust not the engine, at a 150 degrees it having enough trouble trying to build horsepower.

This so call spot is inherent to small block heads which have Siamese exhaust valves in the center of the head and on engines than run at 210 degrees F. with a radiator in racing conditions. Relieving water here would help in is this case, but the center of the head not the back an certainly not overboard. If this scenario does not describe your engine I wouldn't recommend bothering with it. Would you? I hate for people to waste time and money I've done enough of that for all of us. Just make sure when your fixing something that the info you gather is really base on your problem. Most of the time it is the biggest hurdle. Ask ten people the same question get ten different answers " know what mean Vern"

Wow Skatermac u sound like u been doing this a few years Hahaha... Love to talk to u about water...psi....cooling.....prob. First I have a cat with a 600 blown big block. Has a hull mounted pickup I can move up or down. That goes to a sea strainer with a psi dump built in to that. Then it goes to a raw water pump...threw oil cooler...in front of block no t stat...out of intake in to the cmi big tube headers. First got boat and making a good pass blew o ring out of sea strainer at like like 110mph I think gauge was maybe 30psi or more? The gauge is reading water psi from the side of the block. Adj water dump did nothing to realy help. So I looked at water pickup it was deep so I moved it up slowly. Long story short I got the boat now at like 18psi at 115+mph all good but......headers are turning color at the very top from heat for sure and after I slow down from a pass motor will go to about 180 or 190 if I idle. Maybe I am ok like this but don't like my 10g headers looking like this not right. Should I just lower water pickup some? Is sea strainer junk? It is like a $500.00 one.

Thank u.

No real good reason to remove the intake as you are drilling and tapping into the water circuit. I agree that the rear hose should be a small like a -4 as you just want to bleed off any air pockets. You can use on addition line like a -8 or -10 in the front of the manifold if you want to reduce WP. IMO, this project would not be a big priority on the Things to do List.

That's the right way to do it. That will help keep steam pockets from forming in the back of the head. If you have water pressure issues, it's better to dump water before it gets to the engine either at the strainer or from the hose between the strainer and the engine. By draining water at the back of the head, you would be creating a low pressure area right where you shouldn't have one. Water under pressure doesn't boil as easily so fewer steam pockets with more pressure.

Every thing seems to be working good with the crossover and the manifold taped and ran to the front. I also have a merc. pressure releif valve with a lighter spring in it. Water pressure is 16 lbs. at cruse and it goes to about 25lbs. wide open.Tempiture stays right at 150. I also have a 2" short imco lower unit.