How to Fix Mercury Outboard Overheat Alarm? (Explained)

It’s a beautiful sunny day. You sailed with your Mercury overboard engine boat. Enjoying the water with family and friends. Within moments of starting the engine, its overheating alarm sounded! What to do then?

You might think that if the electronic equipment is active, it will get hot and that is normal. What is there to be upset about? But you are wrong. 

The boat’s engines have only one to heat up. Exceeding that level can lead to severe damage to your engine. So as much as possible the cause of engine overheat should be found out and fixed.

In this article, we will learn how to fix a Mercury outboard overheat alarm. We will also discuss the cause of the problem and how to diagnose it.

When or How to Diagnose Mercury Outboard Overheat Alarm Problem?

Several factors can cause your Mercury outboard overheat alarm to sound for no apparent reason. Below are some of the notable reasons that can cause your boat’s overboard to overheat.

Debris in Intake

Plastic bags and weeds are almost always the first things to check when you hear terrible beeping or the temperature gauge turns red. 

The boat’s water pump produces plenty of suction. As a result, it is very easy to suck different types of debris and take raw water in the intake. The growth of marine life while anchoring a boat can also clog water intake.

Clogged Raw Water Strainer

Another reason behind the overheating of the engine of a boat is the problem of water flow in the engine.

Most boats have a water strainer. The strainer catches the harmful substances before they damage your engine. 

Before you go underwater to search for a problem, open your boat’s water filter and check its screen cleanly.

Secondary Water Pump Failure

Land vehicles have a cooling system to cool the engine. Similarly, some boats have a cooling system that is mounted to cool down the engine. 

These cooling systems not only use seawater but also use coolant or antifreeze in a separate cooling system called closed-loop cooling. The mentioned technology is commonly used in large boats with relatively complex engines.

A secondary water pump powered by a gear or belt is mounted on the motor of a boat engine like a car. If you do not maintain the pumps regularly, they can become internally defective.

Failed Impeller

Another major cause of overheating of your boat’s Mercury outboard engine is the failure of the wiring and tear or water pump impeller. 

Almost all types of boats have a pump that feeds the engine water. Inside the pumps is a small rubber impeller that keeps the engine cool by supplying cold water to your boat’s engine, just like a paddle wheel.

Gradually the rubber raw water pump impeller erodes or is damaged by various debris in the water. As a boat owner, you should check the impeller regularly.

Blower Failure

Many boat engine bogies do not have very good airflow. The temperature is controlled by providing additional air through the electric blower. 

These blowers are cooling fans, which draw air from outside and circulate the engine bogie to keep it cool overall.

Excessive heat generated in the engine compartment can cause catastrophic damage to wires, batteries, electronics, and hoses. So before starting the engine you should check to make sure the blower is working.

Internal Engine Problems

An internal engine error can cause Mercury overboard problems. You should never want your boat to have an internal engine problem. 

The help of a professional mechanic is usually required to solve any kind of internal problem of the engine. 

However, if you check and maintain it frequently, you can avoid a variety of costly problems, such as failed gaskets, worn-out heat exchangers, blocked exhaust elbows, and so on.

How Bad Can It Get if Your Mercury Outboard Engine Overheats?

Now, the question may be, how much damage can the excess temperature do to your engine? One thing to keep in mind is that it can cause serious damage to your engine for sure.

The high temperature breaks down the lubricating oil of the engine and increases the friction of the metal with the metal. 

If you notice that your engine has slowed down due to overheating, then you must understand that the engine has been damaged.

If proper action is taken early, that is, if you turn off the engine at the first sign of overheating, there will be no serious damage. 

It is often possible to run idle through a faulty cooling system without damaging the engine. But always be careful. Do not ignore the overheating warning.

Cost of Fixing Mercury Outboard Overheat Alarm

In case of primary problems, you can fix your Mercury outboard overheating alarm without any cost through DIY. 

However, if the problem is serious, you may need to seek the help of a professional. In that case, you have to pay that person his dues. 

On the other hand, if a part is not repairable, that is, it has to be replaced, then the cost will be more or less depending on the depth of the problem.

How to Fix Mercury Outboard Overheat Alarm?

Now we come back to our main task, that is, how to fix an overheating alarm on a Mercury outboard. It is worth mentioning here that we will make our solution based on the problems mentioned in this article.

If There Is Any Debris in the Intake

Pick up the stern drive or outboards. Check that the small grate where the water goes is clean. 

In the case of inboard, jet-ski, and sailing boats you may have to dive into the water to check and clear the water intake.

If the Raw Water Strainer Clogged

The water strainer on your boat is probably located near the water intake. You can reach the bottom of the hull from the water pump only if you follow the hose. Check and see if everything is OK there.

If the Secondary Water Pump Failed

If your boat’s secondary water pump malfunctions, repair it. If the pump is not repairable, you may need to replace it completely.

If There Is a Failed Impeller

Always keep an extra impeller in your boat. You can change the impeller with a simple hand tool as needed. An impeller puller is the most effective tool in this case. 

However, if there is no impeller puller, the work can be completed with a plier and a screwdriver.

If Blower Failure Happened

Open a hatch if possible and if you feel safe to solve the problem mentioned. It will ensure additional ventilation. 

Also, check the fuses as well as the electrical connection. Repair or replace the blower with skilled hands as needed.

If There Are Any Internal Engine Problems

It is better not to do it on your own for any kind of internal problem of the engine. Either contact the manufacturing authority directly or seek the help of a professional mechanic.


Your mercury outboard overheating alarm problem is mainly due to the reasons mentioned in this article. Any kind of normal warning is acceptable, but if the warning is abnormal, it is a matter of concern. 

That’s why you first need to know the reasons why your engine heats up naturally. It is possible to fix the overheating alarm only if the problem is solved properly.

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