Originally Posted by Tim R.
178° at the cylinder head is fine. What temp do you get when you point it at the top of the thermostat housing? That is the most accurate reading you will get of the fresh water side. You could be running normally if you have a 180° thermostat installed.
The new thermostat is sitting on my nav table while I try to sort out whether I have a problem or not. Back to the boat yesterday to take some infrared readings and notes so I wasn't relying on a poor memory. The gauge has no increments between 180-240 degrees so it's a guess to what it is actually reading. After 20 minutes of running the engine @ 2000 rpm under load the gauge read approx 185, the infrared reading at the base of the thermostat housing was 175, cylinder head was 175: After 25 minutes gauge was approx 190, thermostat 181, cylinder head 180: After 30 minutes gauge was approx 210, thermostat 187, cylinder head 187. That was enough for me. I idled the motor for 5 more minutes and shut her down. There is steam coming from the exhaust and although I've noticed this in large cruising powerboats before I've never noticed much if any from my boat in the past. I've removed all hoses and ensured they are not blocked. The impeller checked out OK as did the FW cooling pump. The only thing I haven't removed is the cast iron riser where the exhaust gasses and spent salt water combine. I imagine there must be some blockage at that point. I'm convinced from what others have reported for temperatures and my own readings that I definitely have a problem.