In the spring this year I changed one of my thermostats to 160 degree. (at the time dealer only had one) That engine will run up to about 180 or 190 then the thermostat will open and the temp will drop to about 150 and then build again. It has worked like this all summer. My question is the other engine has a 140 degree thermostat and the engine stays around 150 or 160. I changed to 160 because I read that the engines run better if they run a little hotter. What about drilling holes in the 160 ? or should i go back to 140 ? Also changed both impellers at that time, the old impellers looked good and I have never had an issue prior to changing the thermostats. Any thoughts or experiences ?

Change back to 140*

You need the 140 as Griff states...The raw water going through the block with the 160 has a better chance of building a steam pocket.