It seems to me that you would be better served if you focused on your oil temp and just left your water temp with little or no reading in the gauge.

i would suggest that , somehow you have this thing plumbed incorrectly. first, if you want this motor to last you need real water temp... 150 at an absolute minimum and 170 would be a lot better. second , to me if i read what you write and try to visualize what you have done, it sounds to me like your bypass isn't really a bypass in the truest sense of the word and by that i mean that it originates on the cold side of the tstat and allows free flow to a non pressurized outlet ...in effect bypassing the stat, as the name implies. it sounds to me like your bypass dumps into a blocked passage or into a pressurized side and therefore has little or no flow. if i where standing where you are standing, as a test i remove the stat and put in a 3/4 or 1" blanking plate as i suggested earlier... i would presume that while i may THINK i have a bypass system, for some reason to be determined later, i really don't. i would adjust the size of that hole until i got a nice solid equilibrium at 170 at the speed and load that you mostly operate. once i got it to do that, i might revisit the whole bypass/stat thing until i actually understood as a matter of litteral fact, what exactly was going on.

as of right now, i simply don't believe the true flow path is what you think it is... if it was, it would work. we aren't talking rocket science here. something that you think is one thing is going to turn out to be something else...

what you don't want to do is run it ice cold like you are... all that does is wear out the pistons and rings and fill the oil up with fuel.

Hello Steve-

Well, yesterday I hollowed out the stat. So all I have now is a 1" or so hole. I removed the bypass hose. Ran the boat. Water temp was at a constant 110 degrees. Oil temp ran at a constant 160 degrees. water pressure was 5 - 10 lbs. So, I think Im headed in the right direction. I need to go to a smaller hole in the blank plate. (thermostat) Does someone sell different size plates?

T-stat restrictors Summit or Jegs they are popular with stock cars.
Thumper

Steve, Mercruiser doesn't agree with you. They put 140 degree thermostats in their engines for many years and just went to 160 recently with their fuel injected motors. Mercruiser believes to make these things live, jet them rich and run them cool. Also, if you don't understand how a crossover with a bypass works, why are you telling someone how to set it up?

Thank you....Just ordered. Kit comes with 3 different plates and 3 different size holes.