Easy Step-by-Step Guide of the Most Common Boat Motor Repair

It’s happened to all of us. You’ve planned a day on the water, but your outboard motor isn’t cooperating. Maybe it won’t start, or perhaps you’re out on the water, and your engine is overheating. We’ll walk you step-by-step through the most common boat motor repair you’ll need to do as a boat owner, how to prevent these problems, and how to stay prepared.

Are you asking yourself:

• Does my outboard engine have a dead battery?
• Does my boat motor have a clog in the fuel system?
• Are my outboard engine’s spark plugs bad?
• Why is my outboard engine vibration worsening as I throttle my engine?
• Why is my outboard engine overheating?

If these questions and how to fix them are in play, keep reading.

The Problem: Dead Battery

The Symptom(s):

Boat motor won’t turn on.

The Solution:

1. Remove the battery from its housing. To do this, disconnect the black (negative) cable, followed by the red (positive).
2. Inspect the battery for damage. A damaged battery will need to be replaced. However, if the only signs of wear are corrosion or residue, you may be able to salvage the situation— scrub corrosion away with a wire bristle brush, and see if the battery will hold a charge. Reconnect the wires, starting with the positive red cable. Use dielectric grease on the connections.
3. If the battery doesn’t charge using the onboard battery charger, you may be able to hook it up to your car battery to give it a boost. Let it charge this way for a few minutes, and then try again with your onboard charger.

How to Prevent It:

It’s always a good idea to test your battery and motor before an outing. In addition, stay on top of your regular maintenance schedule and keep an eye out for corrosion.

A final tip: take into account how the weather and your boat usage may affect your battery.

The Problem: Clogged Fuel System

The Symptom(s):

Engine surges and dies; engine sputters.

The Solution:

The most common places that fuel will clog are the fuel filter and the fuel line.

1. First, check the fuel link for cracks or kinks
2. If there are none, there is likely a blockage inside the line or filter. To check the fuel line, first, disconnect it. Use compressed air to force out and gunk or debris that may be causing the problem-- a dinghy air pump is suitable for this.
3. If the fuel filter is to blame, it can easily be replaced with a spare.

How to Prevent It

Bad fuel is the likely culprit here. There may be debris that somehow got into your tank, or ethanol may be the cause. Without a fuel additive, ethanol-enriched gasoline can become unstable. Ethanol prefers to bond with water rather than gasoline. Through the natural process of condensation, your boat operating in a water environment, and accidental introduction of water into the fuel system, water finds its way into the fuel tank.

If there is enough water in the fuel tank, ethanol will leave the gasoline and bond with the water, potentially doubling the volume of the water in your tank. You can end up with water enriched with ethanol rather than ethanol-enriched gasoline. This phase separation, as it's called, can lower your fuel octane and sharply reduce your outboard’s performance. Ethanol enriched gasoline can also form thick gunk that can clog the fuel filter, but why?

As it turns out, Methyl tert-butyl ether (MTBE) is an additive in some non-ethanol gasoline. Its job is to increase octane and oxygen levels in gasoline and reduce pollution emissions. However, MTBE can build up in your outboard’s gas tank. When ethanol combines with MTBE formulations, gunk builds up in the gas tank. The usage of both non-ethanol formulations (containing MTBE) and ethanol enriched formulations can lead to the formation of thick gunk that can clog your outboard’s fuel filter and other fuel system components, leading to sputtering or worse.

Ensure that you’re adding the appropriate type and amount of fuel stabilizers & additives to your tanks to protect your motor efficiency and longevity.

Alternatively, if parts of the fuel system are old and need replacement, small pieces may be breaking off and leading to clogs. Keep up with regular maintenance to prevent this.

The Problem: Bad Spark Plugs

The Symptom(s):

The boat motor is losing power; boat motor won’t start.

The Solution:

Fuel, dirt, and other substances can foul a spark plug and prevent it from igniting. Remove the plugs to inspect them and their wires. Most residue can be removed with a rag; however, significant buildup will need to be scraped off. Do this very carefully— damaging the plugs will render them useless.

If your plugs are beyond repair, you’ll need to replace them. You’ll need only four tools for this repair: a torque wrench, spark plug socket, dielectric grease, and spark plugs.

1. Disconnect the negative terminal from the battery and remove the spark plug boots.
2. With your engine in neutral (and your keys safely out of the ignition), use the torque wrench and spark plug socket to remove the plugs.
3. Apply dielectric grease to the tips of your new spark plugs and insert, carefully tightening them by hand. You may then use the torque wrench to further tighten the plugs according to your owner’s manual specifications.
4. Apply dielectric grease to the spark plugs’ ends and secure the spark plug boots in place.

How to Prevent It

Spark plugs should be routinely checked and replaced. We’ve got a fantastic guide to marine spark plugs that outlines the answers to any question you may have.

The Problem: Bad Propeller

The Symptom(s):

A vibration that intensifies with increased speed; the boat motor starts but won’t stay running; you’re going through gas more quickly than usual.

The Solution:

There are several causes of propeller problems: tangles, damage, or incorrect parts. Fortunately, it's a simple boat motor repair that requires little prior experience.

1. Check your propeller. If fishing lines or other materials have wound around your prop, it may be causing immobility. Carefully remove stubborn materials from the propeller using pliers, a knife, or another appropriate tool you may have handy.

2. If this doesn’t appear to be the issue, you’ll need to inspect the propeller for signs of wear and tear. A damaged propeller will have bends, dents, or other obvious signs of wear. These dings can negatively impact how efficiently your motor propels your boat.

Unfortunately, a damaged propeller will need to be replaced. While suitable for some boaters, aluminum propellers are far more susceptible to damage and may not be an ideal investment for frequent boaters.

If neither of these issues seems to be the source of your trouble, it’s possible that you simply have the wrong propeller! When selecting your prop, you’ll want to consider the pitch and diameter. The pitch refers to the distance that one rotation of the propeller will move the boat. Too much pitch will result in difficulty coming onto a plane; without enough, your prop will struggle to provide adequate acceleration.

How to Prevent It

A bad propeller is best prevented by routinely checking your propeller for dings, dents, and other signs of damage before you head out on the water. Getting stranded is never a good feeling!

The Problem: Overheating

The Symptom(s):

The temperature gauge is in the red; the outboard motor suddenly loses power; the boat motor won’t start up again after you’ve turned it off.

The Solution:

This is another common boat motor repair with several possible causes and solutions. The most common causes are a bad impeller or low engine oil. Or, it could also be caused by a sweltering day with heavy usage. When this is the case, you should simply leave the engine off for a while and allow it to cool. If that doesn’t work, take the following approach:

Low Engine Oil

1. Check your engine oil with a dipstick. Keep in mind that different engines have different requirements at each point of this process. For example, it’s best to check the oil levels of a Mercury outboard while the engine is cool, but a Yamaha may need to be at operating temperature for an accurate reading.
2. If the oil is low, add a little at a time until you’ve reached the recommended level for your engine.

Failed Cooling System

1. You can check for a bad impeller by removing the impeller from the stainless steel cup that holds it. Common signs of damage are deformities, such as bent or curved fins and chunks of rubber missing.

The damage you see may help you pinpoint the issue. If chunks are missing from the impeller, you likely have an underuse issue. When the impeller sits for a long time, it becomes brittle and susceptible to breaks. If this is the case, the impeller needs to be replaced.

2. A deformed impeller is a sign that water is not adequately moving through the cooling system. Friction is causing the impeller to bend. Most commonly, there is something blocking your water intake. Locate the intake and clean out impacted mud, debris, or pollution.

If you have a snake or rod to clean out the intake, you can quickly solve this problem. The impeller will need to be replaced in this instance, too, but cleaning out the intake should prevent future damage.

How to Prevent It

The best way to prevent overheating is, as you may guess, regular maintenance. Be sure that the entire cooling system is fully functioning. Beyond that, it’s important to keep an eye on your temperature gauge and be proactive in preventing overheating.

Need additional info for your Yamaha or Mercury boat motor repair? Reference our handy guide to diagnosing your boat motor problems.

We hope these tips help—Shop PartsVu for your boat parts and maintenance products. Care and maintenance of your marine engine are critical – if properly maintained, it will last for many, many years.

This content was designed to serve as a general guide. Persons who are not familiar with boat motor repair and who have not been trained in the recommended servicing or installation procedures should have the work performed by an authorized marine mechanic. Improper installation or servicing of parts can result in engine or boat damage, personal injury to the installer, or harm to persons operating the product.