Mercury 40 HP 4 Stroke Problems – Engine Maintenance Guide

Mercury 40 HP 4 Stroke Problems and solutions (1)

Made up your mind to go fishing with your favorite boat ride? It seems your 4-stroke engine just has refused to start! If this is your scenario, you have come to the right place!

Sitting around and getting mad won’t be any help to solve this! We show you the fixes here.

So, what are the Mercury 40 HP 4 stroke problems?

Several problems like the incompatibility of the engine, and overheating are there. Sometimes, a lack of fuel efficiency might put your boat into serious trouble. Along with that, you might require more repairing of its different parts. Moreover, its lubrication system needs periodic maintenance.

But that’s not all! However, this article recognizes the common problems that many users have met! And, it delivers feasible solutions.

Let’s not beat the bush around and dig this deeper right now:

Mercury 40 HP 4 Stroke Problems and Possible Solutions

Mercury 40HP EFI WOT

Enough of scratching the surface! Now it’s time to dig out the real problem related to this engine. And, by the end of this section, ideal solutions are at your doorstep.

Problem 1: Incompatibility With Smaller Boats

We know four-stroke engines come heavier than other popular strokes. You can not suit the Mercury 40 HP 4 strokes while adjusting with a smaller boat.

A clear weight imbalance between the boat and engine will be visible due to this misfit. Similarly, Yamaha 25hp 4 Stroke Outboard Problems also draw this incompatibility.


So, always picking the stroke variety is ideal once your boat size is confirmed. We suggest you pick two-stroke-based engines such as Mercury or even Yamaha.

This approach will make your outboard engine choice worthy enough. You are getting much time to think if you should get Mercury 4 strokes or not.

Problem 2: Boat Engine Sputtering Issue and Eventually Losing Power

This issue is so common happening with the Mercury 40 HP 4 Stroke engine. Seeing the engine sputtering out basically indicates the decrement of its strength.

In the meantime, happening this so frequently and repetitively leads us to something more dangerous. Filter problem or spoiled plugs is the key reason behind this.

As a result, sooner or later the boat motor starts losing its power. Engines inside outboards like hang kai outboard issues also indicate having this problem!

Mercury 40 Horse 4 Stroke for smaller boats


Every owner should vent the engine chamber completely before resuming. If you don’t do that, a clogged filter might appear in the picture.

Meanwhile, the odds of mixing between the gas and water- we all know. So, avoid leaving the fuel tank empty. Rather, you need to fill the tank as much as possible.

Users mostly face this condensation trouble after leaving the boat for no operation. And, the more you keep away working on it, the more condensation chances are there.

We suggest you get a fuel stabilizer. After that, leave the tank for 3 or 4 months longer if required! Nothing harmful will be happening. Along with that, you might choose a simple path to prevent sputtering.

Cleaning and changing the spark plugs in time is the key here. Changing these spark plugs should be done according to the motor usage.

Also, don’t forget to change the fuel filter on a regular basis. So, try to reinstall the in-line fuel filter.

What if you have not got one? Then, clearing debris from the filter component will be enough. Along with that, drain the water that was stored earlier.

Before doing that, simply check the manufacturer from where you bought this. Spark plug details will indicate when is the perfect time to replace them.

As the first sign of deterioration appears, you should work from that period. Along with these, keeping a simple toolbox on board carrying basic installation instruments is ideal.

The secondary problem is that sometimes you can just get bad fuel. While fuel is supplied to stations, the last ones receiving the fuel might get trash quality.

As a result, fuel might get stored at the bottom of the tank. And, during the full-fledged operation, it might cause trouble. Along with that, if you leave the fuel tank empty, condensation might start building up.

How to start a Mercury 40

Problem 3: Engine Not Starting

What else can be bigger than engine failures? Well, lack of fuel efficiency comes first in this circumstance in the Mercury 40 HP engines. So, the first thing you should do is to focus on the fuel system.

Also, you can’t understand the pain until turning on the ignition key and hearing no sound. This is quite frustrating indicating your temporary engine failure.

To keep it active and running with full control, the ignition key must produce a sound. However, the absence of necessary steps at the right time might result in long-run engine failure.


Check if any portion of the fuel is being kept in the engine for so long. So, you should not avoid it while finding this circumstance. The fuel inside the Mercury 4-stroke engine might go through a chemical reaction.

As an end product, you might find fuel-producing ethanol. As a result, the prevention of the engine running comes into the picture.

Also, there might be some blockages on filters all caused by dirt. Along with that, chances of line and carburetor blocking exist too.

Second of all, look for the kill button. Check if the shifter is staying at the neutral position or not. Along with that, focus your attention on the starter control.

Sometimes, the ignition switch might relax the fitting automatically. As a result, it might allow the whole switch control to pivot with the key. Simply, get some screws to tighten the retention nut.

Mercury 40 Horse 4 Stroke overheating

Problem 4: Overheating Issue Inside the Boat Engine

Sensitive engines require heat balancing so that they might work for a long time uninterruptedly. Meanwhile, temperature gauges are available to measure the heat.

While you go through checking this device, have you seen the needle rising? Then, your engine might have overheated. Along with that, it confirms having a cooling coil with inadequate water flow.

Unfortunately, most of the outboard engines do not come with radiators. As a result, the chances of relying on water increase. However, it’s not the ideal way to keep your engine cooler.

Suddenly stopping water flow brutally shows how ineffective this is to rely on! That’s where the overheat takes place and results in engine failure.

However, in some circumstances like Yamaha F115 4-Stroke problems also show this characteristic. So, you might find it unable to start. Meanwhile, Mercury outboard engines are more painless to operate.


Among 40, 50, and 60 HP variants, Mercury 40 HP is not that top-notch in performance. And, chances of overheating issues are so common.

First, investigate the cause. Check the raw water input. You might slow down the water flow by the slack hose clamps or burst hoses. It might heal the damaging moisture around the engine.

Mercury 40 Horse 4 Stroke

Problem 5: Required More Potential Repairing

The more parts existing in a system, the more repairs the system might require.

And, we already know building a 4 stroke engine requires more parts behind this for functioning. So, this case is obvious in the four-stroke engine, isn’t it?

Meanwhile, its lubrication system might require more periodic maintenance. Can you guess why? As long as they come with more parts, they might demand more lubricant to cover.


Utilizing a 2-stroke engine might be an option. It might require fewer parts to operate compared to a 4-stroke. And, you will explore more power generation here than a 4-stroke engine of the same horsepower.

A 2-stroke engine utilizes two pistons only for one revolution of crankshaft power generation. As a result, more power generation is possible.

Meanwhile, cooling, sealing, cleaning, serving, and lowering friction are the key securities for moving parts. And, managing such five steps is not an easy task.

That’s why periodic maintenance is a must. So, check these steps regularly whether these have been completed or not.

Mercury 40 Horse 4 Stroke

Engine Maintenance

While these engines can be maintained with basic care and routine maintenance, there are a few things that should be taken into account to ensure optimal performance and longevity.

One important aspect of maintaining a Mercury 4-stroke engine is keeping the oil level consistent. Over time, the oil will wear down and reduce the engine’s performance. It is important to check the oil level regularly and add fresh oil if it begins to drop below the recommended levels.

It is also important to keep an eye on the air filter. If it begins to clog up, this will cause reduced airflow through the engine, which can lead to problems such as rough running and decreased fuel efficiency. It is best to replace the air filter every 6 months or whenever it starts showing signs of wear.

When it comes to cleaning your Mercury 4-stroke engine, make sure to use a quality cleaner that is specifically designed for marine engines. Avoid using harsh chemicals that could damage the engine or equipment. Instead, opt for a cleaner that contains Corrosion Inhibitors or Degreasers. By taking these simple steps, you can keep your Mercury 4-stroke engine running smoothly for years to come!


Mercury 40 Horse 4 Stroke faq

1. Are Mercury 4 Strokes Dependable?

Though Mercury comes with 2 strokes-based variants too, its 4 stroke engines are more dependable. Meanwhile, having an electronic fuel injection system makes this device more reliable. Along with that, you will find it much more fuel-efficient. As a result, hitting the long run can be accomplished.

2. Why Is My Outboard Slow-moving?

Having a spun prop comes first as the key reason behind causing this. It makes the powerboat struggle to gain suitable speed at full throttle. This occurs when the connection between the boat propeller and the mounting shaft gets damaged. That’s why you find the rubber inserts begin revolving alone.

3. Yamaha or Mercury Outboard- Which One Is Better?

Well, both are great at their specific fields. Yamaha comes greener and more nature-friendly outboard between these two. And, it provides two electric motors while Mercury has to offer none. Meanwhile, Mercury is the ideal pick for getting a long-lasting warranty of more than eight years.

4. How many hours is a Mercury 4-stroke good for?

A 4-stroke Mercury engine typically lasts around 3000 – 4000 hours before needing to be overhauled. This means that a four-stroke Mercury engine can typically last for around four years or 1500 hours of use.

Final Words

Now you know the solutions to Mercury 40 HP 4 stroke problems. We’ve tried to cover common problems regarding this.

We hope that you might have got an idea about what you require at present.

Still, if you have any queries, let us know! Thanks for staying put this far!

Related Articles