Typical Reasons Boat Engines Overheat (Solved!)

It is a beautiful day on the water. Your boat is cruising along, and all are enjoying the ride.

The engine overheating alarm starts screaming. What do you do?

Throttle down immediately. Then consider the safety of your boat and passengers. Are you in safe water where you can anchor? Can you idle to somewhere safe?

Now it is time to diagnose and solve the problem.

Diagnosing the Problem:

All marine engines use seawater to cool the engine.

Outboard motors have a telltale stream of water emerging from under the cowl.

Inboard engines often have the cooling water exiting from the exhaust pipe. Look to see if cooling water is circulating through your engine.

If not, check the following:

1. Is the water inlet clogged with weeds, plastic, or debris?
2. Is the seawater strainer clogged?
3. Is the water pump impeller damaged?
4. Is the thermostat stuck?
5. Is there a broken belt?
6. Are there broken hoses or connections?

Let’s Look Under the Hood:

V belts are used only on inboard engines and are often the culprit.

Simply opening the engine hatch will confirm that the V belt is intact. Apply finger pressure to the belt and confirm that it moves between ¼” and 1/2”, the correct tension.

While in the engine compartment, do you smell steam or smoke? Check the coolant level and look for broken hoses. My engine overheated when a simple hose clamp rusted through. The hose fell off the water pump. It was easy to find and fix.

Check not only the coolant in the overflow reservoir but remove the heat exchanger cap and be sure it is full of coolant. But don’t touch the cap until the engine has completely cooled down!

Water Pumps & Diagnosing Them:

Water pumps are often the cause of overheating and a bit more difficult to diagnose.

Outboard motors have a rubber water pump impeller in the lower unit. If there is no water coming out the telltale stream, first check the water intake grate or screen in the lower unit in front of the propeller.

Tip the engine up and be sure the intake is not clogged. Remove the engine hood and see if any hoses are broken or disconnected.

Look for cracks in the engine or water leaking from anywhere. If you can’t find an obvious problem, the water pump impeller is most likely faulty. This can not be repaired on the water. Take a tow to where a mechanic can work on your motor.

Inboard engines often have two water pumps. The seawater pump has a rubber impeller similar to an outboard motor. The coolant pump is on the front of an engine, just like a car. Replacing the seawater impeller while on the water is possible with a few hand tools.

Be sure to carry a spare.

The coolant pump is bad only if it is leaking or if there is play in the bearings. Grab the pulley and give it a shake to test for play.

Overloaded Engines can Cause Overheating:

Several factors can cause overloading:

• Oversize propeller
• Overloaded weight on the boat
• Running at too high a speed.
• Improper operation

Overheating from excessive load will show up only at higher throttle settings.

At low speeds or idle, the engine will cool properly. If overheating only occurs at the higher throttle, look at one of the above factors for the cause.

Running at sub-planing speeds, plowing through the water places a high load on the engine. Either plane out or run at a slow speed, nothing in between.

Faulty Temperature Gauge or Alarm:

But don’t count on it. Be sure to check your cooling system thoroughly before suspecting the gauge.

Aiming an infrared thermometer at the hoses and cylinder head can determine if you truly have an overheating problem. Anything over 190F or 90C indicates overheating is occurring.

How Bad Can it Get if your Engine Overheats?

Overheating can cause serious damage to your engine.

High temperatures break down lubricating oils and cause metal-to-metal grinding. If your engine slows down from overheating, you have caused some damage. 

If you shut an engine down at the first sign of overheating, there will be no damage.  Running at idle is often possible with a faulty cooling system without causing the engine harm.

Use caution, and don’t ignore that warning buzzer!

How Do I Prevent Overheating?

Get to know your engine.

Study the maintenance manual. Learn what components are essential for proper cooling.

Carry spare V belts, water pump impellers, hoses, hose clamps, and coolant, and carry basic tools to do simple repairs.

An inexpensive infrared temperature device is a handy tool to have. Check for telltale cooling water or water exiting with the exhaust every time you start the engine. Replace water pump impellers every two years.

Proper maintenance and basic knowledge of your engine’s cooling system will ensure trouble-free operation and enjoyment of your boat.