Well your temp is low on the gauge, so if you want to check first without taking anything apart, get an IR gun from Radio Shack or Sears and let the motor warm up. Then shoot the temp right under the thermo housing on the intake manifold. It should be around the rated opening temp of the thermo, which for most late model Mercs and Volvos is 160, so your reading could between 155-165. Lower than that, and it's sticking open a bit. Using the IR temp gun eliminates any question about the accuracy of the gauge. BTW, my old gauge is pretty much right on with the temp gun.
In that pic, you are at 3000 rpm, right and if that's a true reading it has to be stuck open. Mine when it sticks a little, will idle at 100-120, then heat up to 160 on plane, then cool down to 120 again when I slow down to idle. So if it's that low at that speed I'm guessing it's stuck wide open not just not closing all the way.
To check for sure, I get a pot that my wife is not going to use anymore (if you don't want to get hollered at) to check it. I then remove the stat from the thermo housing. If its a Volvo, they use the same housing as OMCs, you turn over the housing look inside and there is a rubber ring you have to pry out with a small screwdriver to get the stat out. I take a length of coat hanger wire and use that to hold the stat in the pot so it doesn't touch the bottom or sides. Then fill with water and use a radiator thermometer to check the water temp while heating it to a boil. You could also use the IR gun to measure the temp. It should be all the way closed and then start to open at 155-160 and be fully open at 165-170 or so. Let it heat up and cool down a few times to make sure it closes and opens. Re-assemble if OK, if not OK buy a new stat and re-assemble. OMC gasket sealer or similar on both sides of the gasket and the bolts so they will come out. I have done this even with the boat in the water on my salt water mooring. I just stick a wood plug in the raw water intake hose so water does not leak in the boat. I change em every 2 seasons, sometimes if I put it in the boiling water and cycle it open and closed a few times they work OK again. Next engine will be closed cooled no more of this raw water cooled nonsense. These engines are designed to run at 195-220 or so running them at 120 is not good for them. 160-170 is Ok but 180 with closed cooling is better.
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