3 Problems with Mercury Outboard Motor (& How to Check and Repair)

Mercury outboard Problems

Known for their superior horsepower range and efficient engine performance, it’s undeniable that Mercury outboards are leaders in the industry. 

Like any marine equipment, these outboard motors experience problems from time to time. 

Mercury outboard motor problems can be frustrating, not to mention the hefty costs of professional repair services. 

Therefore, knowing how to troubleshoot your motor and addressing these issues quickly is crucial in preventing unexpected breakdowns or severe damage.

Whether experiencing fundamental motor issues or complex engine failure, this guide is your ultimate companion! Let’s explore the causes and fixes for common Mercury outboard motor problems and some helpful tips to prevent them. 

1 – Overheating Issues

Outboard overheating causes idling, or worse, sudden loss of engine power. In such circumstances, your outboard motor will continuously beep or alarm if it’s starting to overheat. 

Constant overheating affects overall performance and efficiency in the long run. It may damage engine components like cylinder heads and valves. Additionally, it increases fuel consumption because it puts the engine under more pressure.

What Are the Common Causes of Overheating, and How to Fix Them?

Here are the possible reasons why your outboard motor overheats. These problems are mostly prevented through regular maintenance:

Thermostat Failure or Blockage

A faulty thermostat fails to open or close to facilitate the coolant or water flow that regulates the motor’s temperature. This often happens because of corrosion or build-up of sand or debris.

To check your thermostat, you can unbolt the housing to remove it, then immerse it in boiling water to see if it opens properly. It should also close after you remove it from the water and let it cool. It’s a telltale sign that it needs replacement if it doesn’t. 

Water Intake Clogging 

Sometimes, trash, weeds, and foreign objects block the water intake, limiting the entry of water that cools the engine. Inspect for clogs and remove them. Cleaning your water intake regularly is the best practice to prevent this recurring issue. 

Bad Impeller

Remove and replace worn-out or damaged impellers to ensure efficient water flow. Impellers are fan-shaped rotating devices that draw water in and out of the engine. Since most impellers are made of rubber, they tend to wear out over time. 

2 – The Motor Fails to Start or Suddenly Shuts Down

When your outboard motor fails to start or unexpectedly turns off, the causes could be the ignition switch, battery, and fuel filter problems.

How to Troubleshoot a Mercury Outboard Motor?

Before you jump to more complex repairs, it’s best to address minor issues that could be causing the problem. Follow this quick troubleshooting guide to help you out:

  1. Check the kill switch attachment. Attach and reattach the switch and see if it solves the problem.
  2. Keep your gear neutral before starting your engine again.
  3. Keep your gas tank vent closed to prevent air displacement. Ensure there’s enough fuel, too. 
  4. Inspect for tangled objects or damages in the propeller.
  5. Adjust the carburetor.
  6. If your battery has a switch, try turning it on and off. 
  7. Pull-start the emergency cord.

Why Is My Mercury Outboard Motor Not Starting and How to Fix It?

Your outboard motor is probably failing to start because of the following reasons:

Defective Solenoid

If there’s a click when you’re cranking the engine, but it won’t start, then the start solenoid may be the problem. The solenoid transmits voltage to the starting motor, which may fail due to wire disconnects or shorts. They’re usually located on the right side of the engine, right next to the starter.

Use a voltmeter to test the solenoid and replace the system if it shows abnormal readings. However, if the reading is within range, then the problem could be from the battery.

Battery Problems

A dead battery or faulty wiring often causes sudden engine stops.

Remove the battery from the housing by disconnecting the negative (black) cable before the positive (red) one to solve this. Using a wire-bristled brush, remove debris build-up or corrosion. 

After doing so, please charge the battery and reconnect it by starting with the positive cable. 

Also, check if the fuse is damaged and replace it. If not, the fuel system may be the source of the problem. 

Bad Fuel Pump, Fuel Filter, and Other Fuel System Faults

Start by inspecting your fuel filter and hoses for clogs and other defects. Do the same to your diaphragm, valves, and injectors. 

Ensure all connections are secure and replace or repair all leaks, cracks, and damages. 

Most importantly, check for water contamination in your fuel tank. The presence of water can be due to poor-quality fuel, condensation, or defects in the fuel tank. 

Drain the contaminated fuel and replace it with a new one to solve this. You can also add fuel additives to prevent water problems in the future. 

Water accumulation significantly impairs the fuel pump, too. Rapid fuel usage is a significant symptom of failing fuel pumps.

Defects in Spark Plugs or Ignition System 

Caution: Keep all flammable substances away when working with your ignition system. The process involves heat and sparks that are potential sources of fire hazards. 

First, clean your spark plugs and replace damaged parts. Next, check for the ignition spark using a spark gap tester. If the spark is good, but you still can’t start your motor, adjust the gap, check the oil level sensors and valves, and repair defects. 

3 – Propeller Damage or Failure

If you’re experiencing excessive vibrations or when your boat barely moves even after dropping the throttle, you’re probably dealing with damages or loose bolts in your propeller. 

Additionally, spun hub failure is one of Mercury outboard motors’ most commonly encountered problems. When this happens, the propeller blades can no longer spin properly, resulting in poor acceleration. This may also cause your blades to bend.

The propeller hub fails due to harsh impacts or deterioration, especially for old rubber hubs that harden over time. Poor maintenance and hitting underwater obstacles like rocks could be the potential causes. 

It’s best to carry around extra hubs or props in case of sudden failure in the middle of a trip.


Often, common mercury outboard problems happen due to poor maintenance and deterioration. Given that fact, constantly cleaning and checking your outboard motor for damages is good practice in preventing breakdowns that may result in expensive repairs. 

If your motor fails to start, troubleshoot by checking kill switches, battery, and fuel condition since not all malfunctions require heavy repair.