An engine’s thermostat (or “temperature regulator”) regulates the jacket water temperature to keep the engine running at a normal operating temperature, usually around 185 degrees F.
If the jacket water becomes too hot, the thermostat “opens” to allow cold water in from either the keel cooler or heat exchanger. When the jacket water returns to a normal operating temperature, the thermostat “closes” to keep the cold water out. An old or deteriorating thermostat, however, may not recognize when to close off the cold water and subsequently overcool the engine. Overcooling can damage an engine as much as overheating because the parts don’t expand properly, and the engine is allowed to run under any load at substandard temperatures.
The biggest problem associated with running too cool is excessive carbon buildup around valve guides and behind piston rings. Other problems include combustion contaminants in the oil and the presence of sulfuric acid, which can attack the rings and valve guides. This can cause excessive wear and shorten the lives of these parts by up to 80 percent.
To get the most out of your engine, change your thermostats annually as part of your routine maintenance schedule. You can also determine thermostat wear by monitoring the coolant temperature under load. If it’s running below 185 degrees F, change your thermostats. (Cat engines usually have two or four thermostats per engine. If one fails, change all of them.) The advantages to a regular thermostat changeout is increased fuel efficiency, longer life between overhauls and increased engine performance.