Ran my 2008 225 last week in 55 degree water, my qeustion is how high should the thermostats allow the idle temp to go up before dropping into range. Ran mine at idle it went up to 182 before comming back down to 162. think these thermostats are set at 143 degrees to start opening and should hold the operating temp between 155 and 165. but 20 degrees on the high side seems alot. Does it sound like they need to be replaced? Run the motor at 4000 rpm get temp down to about 155 and if you back it off below 2000 rpm it will climp into that 185 range, maybe the thermostats not opening at right temps in the lower RPM. this is also in 55 degree water, so a little concerned what it will do in 80 degree water.
A thermostat only regulates water temperature in its own area of the motor block. Temp sensors are located elsewhere and are not submerged in water sothey record the temperature of their engine surroundings. At various loads and rpm, your motor will see temp fluctuations depending on conditions. If the over-temp warnings to not activate, your motor is running OK.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.