boat running cool

Old thermostat may have been the wrong one. The thermostat that should be in the engine is a 143 degree.

Mike,
The old T stat may be stuck so it's not a bad idea to get a new one in there. The 140 degree is the correct temp for a raw water cooled engine but that doesn't mean they run at that temp. Usually 20 to 30 degrees warmer than the rating of the T stat but that depends on how hard you're pushing the boat.

Mike,
You're really not running that cold. Just on the cool side! Engine efficiency suffers slightly. Replacing the stat is simply opening up the housing, cleaning up the gasket surface, dropping the new stat in and then closing up the housing. 1/2 hour tops.

Thanks for the help guys! Mike

Wax pellet
A car engine thermostat.An automobile operating on an internal combustion engine requires a thermostat to regulate the flow of coolant. This type of thermostat operates mechanically. It makes use of a wax pellet inside a sealed chamber. The wax is solid at low temperatures but as the engine heats up the wax melts and expands. The sealed chamber has an expansion provision that operates a rod which opens a valve when the operating temperature is exceeded. The operating temperature is fixed, but is determined by the specific composition of the wax, so thermostats of this type are available to maintain different temperatures, typically in the range of 70 to 90 °C (160 to 200 °F).

Modern engines are run hot, that is, over 80 °C (180 °F), in order to run more efficiently and to reduce the emission of pollutants. Most thermostats have a small bypass hole to vent any gas that might get into the system (e.g., air introduced during coolant replacement). Modern cooling systems contain a relief valve in the form of a spring-loaded radiator pressure cap, with a tube leading to a partially filled expansion reservoir. Owing to the high temperature, the cooling system will become pressurized to a maximum set by the relief valve. The additional pressure increases the boiling point of the coolant above that which it would be at atmospheric pressure.