15. What would cause the temperature to go above 180 degrees in my late model engine?

There are three general causes of overheating in late model engines: 1) a restriction somewhere in the system (by far the most common cause), 2) a dirty or malfunctioning thermostat, or 3) a malfunctioning water pump.

In terms of restrictions, if you have just acquired an engine that appears to have had little or no preventative maintenance performed on its cooling system, the restriction could be partially blocked water jackets in the block, head, or manifold. In this case, a good flushing is in order.

If the engine has been cooling satisfactorily for several years, the restriction is more likely on the order of seaweed in the raw water intake, or a small piece of water pump impeller lodged somewhere in the system, frequently in one of the elbows on the pump itself. These pieces of impeller can be from a failure that happened several years earlier, and they are particularly frustrating in troubleshooting efforts. They can flop around in an elbow so that for a while they will let water pass quite normally, and then (without warning) they can as easily flop closed and cause a rapid overheating condition.

In the case of thermostats, they usually simply need to be cleaned. We recommend soaking them in vinegar for a day or two, or diluted household muriatic acid (half water and half acid). To have the best chance of avoiding trouble, thermostats in raw water cooled engines should be cleaned every year or two.

As a general troubleshooting procedure, it is useful to evaluate the amount of water leaving the boat through the exhaust system during the actual overheating condition. If the flow of water out of the back of the boat is relatively normal, the water pump is clearly working, and water is passing normally through the bypass loop, the manifold and the exhaust system itself. In this case, you can limit your focus to potential restrictions that would prevent water from flowing through the block and head, such as a blockage in the “T” fitting in the water jacket side plate or a stuck thermostat.

If overheating is accompanied by reduced flow out the back of the boat, you will have to extend your investigation to include water pump failures and restrictions that would affect total flow. These restrictions include blockage of the raw water intake fitting, pieces of impeller in the discharge elbow of the water pump, blockage in the rear fitting of the manifold, etc. - Updated: November 4, 2003