Outboard Cooling System Maintenance Tips

Outboard motors need their cooling systems well maintained to prevent overheating, and for optimal performance while the engine is operating out on the water.

Outboard cooling system maintenance

Knowing basic cooling system parts and how they work is essential to the longevity of an outboard motor. Not everybody is a mechanic, but just knowing about a few basic things like clogged water intakes, and failing thermostats and impellers goes a long way toward addressing cooling system issues before they cause serious damage to an outboard.  Here are some simple things to know about outboard cooling systems and preventative maintenance. 

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Outboard Cooling System Types

Outboards typically use one of two types of cooling systems: open (or raw water) cooling, or closed-loop cooling. Open cooling systems draw water directly from the body of water they operate in, and circulate that water through the engine to cool it. 

Outboard open cooling system

Closed-loop cooling systems, which are far less common, use a coolant mixture to keep the engine cool. Most outboards use raw-water cooling systems to circulate fresh water or saltwater into the engine. For the purposes of this article, we’ll be focusing solely on open cooling systems. 

Water Pump

The water pump is the cooling system component that circulates water through the engine, typically found atop the lower unit (or gearcase) of the motor. Water enters the outboard’s lower unit through inlets, then the water pump draws it through the intakes and up into the housing, and sends it through a tube up to the powerhead to circulate through the system’s cooling passages.

Outboard cooling system maintenance water pump

While you have to remove the lower unit to access it, the water pump is not difficult to repair or replace. More often than not, the problem with the water pump is impeller failure. The impeller is easy to replace, but it’s best to just get a water pump kit and replace the entire assembly. 

Because open cooling systems carry the water the boat is navigating in, salt, sand and debris often get sucked in and start eating away at the impeller and other parts of the water pump, which is why servicing the water pump or at least changing out the impeller once every season is recommended. 

Outboard cooling system maintenance impeller

Never wait too long to replace a water pump impeller, because once it becomes brittle, pieces will start breaking off into the engine and wreak havoc on it. It takes roughly an hour to remove the lower unit, replace the water pump, and then re-install the gearcase, so don’t put it off when it’s time.

Gearcase Removal

We mentioned removing the gearcase to access the water pump, and this involves tilting the outboard on a work stand, then disconnecting the shift shaft and the shift cable.

Outboard cooling system mainteannce gearcase removal

You also need to remove the lower unit mounting bolts on each side of the engine. Depending on the model, you might also have to remove the trim tab to access some bolts. Usually detaching the lower unit requires some maneuvering, but before you remove the last bolt or nut holding it in place, be ready to catch the gearcase if it happens to drop right off. 

When reinstalling the lower unit, you want to make sure the drive shaft, shift shaft and especially the water tube are positioned properly. If the water tube misses its connection, the engine won’t receive water and will therefore overheat. After reinstalling the lower unit, start the engine to make sure it’s pumping water, and running at the correct operating temperature.

Thermostat

The thermostat is the part of the cooling system that regulates temperature. This component is very easy to replace, and it can’t be repaired. 

Outboard cooling system maintenance thermostat

Replacing the thermostat is just a matter of removing its housing, then swapping out the old thermostat with a new one. Before putting the new thermostat back onto the motor, it’s recommended you clean out any gunk from the housing, scrape out all the old gasket material, and replace the housing’s gasket, diaphragm and spring.

Prevent Engine Overheating With Proper Maintenance

Open cooling systems on an outboard are subject to all kinds of obstructions such as seaweed, algae and even floating trash, which can clog or obstruct the water intake port. 

Outboard cooling system maintenance water intake

If any part of the cooling system gets clogged, it puts the engine at risk of failure. Besides checking the water intake ports, another tell-tale sign of good water flow is … well, what they call the “tell-tale” (or “pee stream”) of the outboard. You’ll know there’s blockage or clogging somewhere in the cooling system if the tell-tale isn’t “peeing” (not spurting) a steady stream of water. 

Outboard cooling system maintenance tell-tale

Keeping an eye on things like steady waterflow from the tell-tale, clearing obstructions from the water intake, and doing regular maintenance on key cooling system components like water pumps and thermostats are all simple ways to prevent overheating. Flushing the engine after each ride is also recommended to remove anything that could obstruct the flow of water.

Inspecting cooling system components such as impellers, hoses and seals regularly, and replacing them as needed will help keep your outboard’s cooling system and the engine itself performing at its peak.

 

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