If you're familiar with automotive cooling systems, the cooling systems found in marine outboard engines are noticeably different. Road vehicles use a "closed loop" cooling system. In other words, the coolant never exits the system, at least under normal circumstances. The same coolant cycles through the system until it's time to replace it.
On the other hand, outboard engines use "open loop" systems. Instead of requiring you to add coolant, your boat's engine pulls freshwater in for cooling. This design, while efficient, places the cooling system under additional wear. As a result, you'll want to complete these three cooling system maintenance steps during your annual pre-winter service.
1. Cooling Passage Flush
Since your outboard engine uses water from the environment, it faces continuous exposure to contaminants such as salt, silt, and organic matter. These contaminants can clog cooling passageways, cause internal corrosion, and shorten the life of your motor. As a result, it's a good idea to flush your engine with fresh water as often as possible.
Even if you don't perform a cooling passage flush with each usage, performing this service is critical before putting your boat away for the winter. There are multiple options for flushing most outboards, including using a garden hose adapter to flush from the top or flushing from the foot while the engine is running.
2. Impeller Inspection and Replacement
The impeller is the rotor on your outboard's water pump that draws fresh or salt water into the cooling system. Most outboard engines use plastic impellers which can wear down over time or even break. A failed impeller can cause your engine to overheat, and shards from a broken impeller can clog your cooling passageways.
Most manufacturers list impeller replacement and service intervals, but it's important to inspect yours at each pre-winter service. If the impeller seems worn or damaged, you should replace it immediately. It's also a good idea to check the water pump housing and backing plate for signs of wear or damage while inspecting the impeller.
3. Thermostat Inspection and Replacement
Since your thermostat deals with the same contaminants as other parts of your outboard's cooling system, it will inevitably wear out and fail over time. As with the water pump impeller, most manufacturers will provide a recommended service and replacement intervals. However, checking your thermostat with each pre-winter service can save you time and trouble later.
Fortunately, testing a thermostat is relatively straightforward. If you have the time, you can test your thermostat as part of your routine service procedure. A quick test will help ensure that you don't suffer any unexpected failures while out on the water.
Reach out to a company that provides outboard services to learn more.