Troubleshoot an Overheating Engine

There are numerous causes for an overheating marine engine. Here’s what to look for.

October 6, 2021
Exhaust elbow and impeller
A scaled or rusted exhaust elbow (left) can slow down the water flow through the engine, leading to insufficient cooling, and overheating. Inspect the impeller to make sure it’s intact (middle), and that it turns as the engine is turned over. Cams wear evenly, so it’s often difficult to determine how worn they are, unless compared with a new version (right). Steve D’Antonio

Recently, while helping a client resolve an overheating problem, it reminded me of the importance of having a clear understanding of the cooling system and where it can fail. Overheating is a common malady to a sailboat’s propulsion system or generator, and is stressful to both engine and crew. A serious overheat can result in costly damage, including warped cylinder heads, leaking head gaskets and scored cylinder walls. It can also be costly. Either way, it’s a fate well worth avoiding, which fortunately is relatively easy.

Overheating Scenarios: There are two means by which an engine or generator might overheat: chronic (and acute chronic) and acute. Chronic overheating typically occurs when moving past a given engine rpm; as you do so, the temperature gauge slowly climbs toward the red. The “red” zone, by the way, might not actually be red on your temperature gauge. (Not all engines have one; if yours doesn’t, it can be added to any engine.) The red zone varies for different engine manufacturers and is dependent on the engine’s thermostat; typically, anything above 195 degrees F begins to reach the level of concern, while many overheat alarms are triggered at 205 degrees F. Once the throttle is pulled back, the temperature-gauge needle falls back to its normal position. Chronic overheating is often accompanied by white, steamy exhaust, not smoky.

Acute overheating occurs without warning, with the gauge rising rapidly, often regardless of rpm. While both types are problematic, chronic overheating is, in some ways, more of an issue because the operator is lulled into a false sense of security. If greater power is needed—to extricate the vessel from a grounding or when beating into a head sea —the engine will not ­provide the power that’s needed without overheating. In the case of acute overheating, the ­problem must be identified and resolved.


Common Causes: It’s best to take a methodical approach to resolving overheating situations; first, try the easiest potential fixes. Begin with the water supply. Is the strainer clear, including the one on the hull if you have one? When running, is the engine pumping water out with the exhaust as usual? If you aren’t sure, you can check flow by closing the raw water/intake seacock, opening the strainer lid, and then slowly opening the seacock: Water should flow freely (and the engine shouldn’t be running).

Next, check the raw-water pump by removing the impeller cover. Is the impeller intact, with no cracks or missing blades, and does it turn when the engine is cranked (without actually starting, if you can help it)? If belt-driven, make sure the belt is not slipping. Is the inside of the cover plate smooth? If it is grooved or scored, it must be replaced. Is the cam—the part that compresses the impeller blades, affecting the pumping action—worn? You might not be able to tell unless you do a side-by-side comparison with a new one.

Next, remove the end caps of the heat exchanger, and check for debris or clogged or scaled tubes (the thinnest scale accumulation can have a dramatic effect on the ability of the heat exchanger to transfer heat). If scaled, the heat exchanger will require flushing with a descaling solution.


Next, check the injected elbow. These are notorious for becoming clogged with corrosion and acting like a bottleneck, a restriction that slows down water flow through the entire raw-water cooling system. One test you can perform to determine whether this is the culprit involves temperature differential: What is the difference between the temperature of the water entering the engine (i.e., seawater temperature) and that leaving the engine at the exhaust outlet? It comes as a surprise to many that the delta should not exceed about 20 degrees F. If it’s greater, there’s a possibility you are dealing with a restriction in the exhaust elbow, or the water supply to the elbow.

Finally, if your engine does suffer a serious overheat, it’s a good idea to replace the coolant and flush/descale the closed (coolant) side of the cooling system. Overheating can permanently degrade the coolant while leaving an insulating film on the heat exchanger tubes.

Steve D’Antonio offers services for boat owners and buyers through Steve D’Antonio Marine Consulting (­


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