Engine Overheating Q and A

By Andrew Spaulding, Crowley's Yacht Yard

Email: andrew@crowleys.com

Q: What do I do if my engine overheats?

A: Continuing to run a marine engine that is overheating is bad…worse than running it in a car. Due to the location of marine engines – enclosed in a boat – there is little air flow to help cool the engine after the combustion air is sucked into the engine. This means that a marine engine will overheat to extreme temperatures very quickly. An extreme overheat can warp the engine heads, crack manifolds, and do other serious permanent damage to the engine. So, if your engine overheat alarm goes off, shut it down as soon as possible. One, this will minimize any damage. Two, this will allow it to cool down as quickly as possible which will facilitate the troubleshooting process.

The engine cooling system on your boat is one of two kinds. If your engine is “raw water” cooled, the water that is used to cool the engine is pumped directly through the engine’s water jacket (voids in the engine block that allow the passage of cooling water). If your engine is “fresh water cooled”, the cooling water goes through a heat exchanger that cools the antifreeze that runs through the engine block. This method of cooling is the same as the one in your car, but instead of an air-cooled radiator to keep the antifreeze cool your boat has a water-cooled heat exchanger.

Either way, the cooling system in your boat has several components that allow it to work properly and a failure of anyone of these can lead to an overheating condition. Since we are discussing a system, it makes sense to start at one end and go to the other. Once any water restrictions are resolved, test the system by starting the engine and making sure that you have proper cooling water flow. Run the engine up to operating temperature and check that the engine doesn’t overheat. Do this before getting underway again. Remember, many times overheating issues are caused by more than one water flow restriction, so don’t get discouraged if fixing the first restriction you find doesn’t solve the problem.

Cooling water (or raw water) enters the system through the hull of the boat, either through a thru-hull and seacock valve or a under water drive unit. The incoming water can be reduced by marine growth, mechanical failure or floating garbage, so the first troubleshooting step is to make sure enough cooling water is entering the boat. My rule of thumb is when you pull the hose off on the inside of the boat to check the flow, the amount of water coming in should be scary…if it dribbles in you don’t have enough. Oh...one more bit of advice; before you do this make sure your bilge pump works!

On most boats the next component of the cooling system is the raw water strainer. Often the strainer can get clogged with plastic bits or marine growth. Before you open the strainer to clean it, shut the seacock to stop the incoming water flow. Clean the strainer basket out and reassemble the strainer. Once you reopen the seacock, check the strainer for leaks, and tighten the lid or basket housing if necessary.

After the strainer, the cooling water flows to the raw water pump. Typically this pump is a neoprene vane impeller type of pump. Remove the cover plate and inspect the impeller for damage or missing blades. Also, there is a cam in one side of the pump housing that compresses the vanes as the impeller rotates. This is what causes the rotating impeller to pump water. A worn cam can cause the pump to push an inadequate amount of cooling water without any other visible signs of trouble. 

If there are missing blades (this is typical result of overheating even if this wasn’t the original cause of the overheat), it is important to track down the missing blades in the rest of the cooling system. The torn blades can be in small pieces which can lodge themselves in the cooling plumbing causing additional water restrictions. Be sure to check in the pump itself since we’ve seen blades stuck everywhere including in the pump inlet. Even if the broken blades are not causing a restriction at the moment, it is likely that they will move in the future with engine vibrations and water flow to a spot where they will cause a problem. This can happen months or even years down the road. 

Once any missing impeller blades are accounted for it is on to the heat exchanger if you have one. There are many small tubes in the heat exchanger and they can get clogged over time from corrosion or marine growth (or impeller blades). In some heat exchangers the tubes are so small that they can get clogged by mud and/or sand (careful running the engine in very shallow water). Some tube bundles can be removed for cleaning and some have to be cleaned in place.

Before and after the main heat exchanger, it is possible that the cooling water runs through additional heat exchangers (transmission, for example) or other parts of the engine so be sure to check the flow at each hose connection as the water flows in and out of various components. Once the job of the cooling water is done and it is on its way out, it goes through the mixing elbow where the cooling water gets mixed with the exhaust. Water flow through the mixing elbow can be reduced by soot and/or corrosion. 

From the mixing elbow the water flows out of the boat through the exhaust on boats with a “wet exhaust”. Some boats have “dry exhaust” systems where the water is removed from the exhaust or in some applications the water is never mixed with the exhaust. In these cases, you should be able to follow your boat’s plumbing to the cooling water exit. 

Hopefully, one trip through the system gets you back underway. If not, it might be time to call in the experts. As always, we are here to help…call anytime.