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Mercury 90hp 4 Stroke Problems – Full Description of the Problems

The world of engines and motors is too hard to understand for everyone. There can be thousands of moving parts in an engine.

But, what are the mercury 90hp 4 stroke problems?

There are a lot of problems with this engine. The most common problem is the engine doesn’t start. Also, the engine doesn’t hold the speed for long. Although, most of the problems are related to water being pumped into the engine. Also, if your engine is sitting idle for too long, keep the fuel out.

But wait! There is more about the problems. Below we have given a full description of the problems. Also, we have given the required solutions. So, in order to learn, jump below!

What Are the Problems With the Engine

mercury 90hp 4 stroke

Engines are really complicated. To some extent it is over-engineered. But in order to move, all the parts are really necessary. There are thousands of moving parts in an engine. And working collaboratively, the engine creates momentum.

But everything has problems. But understanding an engine can be tough for everyone. Since there are so many parts in an engine, it can be confusing. Moreover, in an engine like mercury 90hp 4 strokes, you’ve to think about water as well.

Since this engine is made for boats, you have to work with water as well. Engines and motors don’t run on water. So if water gets in the engine, it can cause problems. Problems such as that, are really common.

You can also read about Mercury 115 Pro XS in our guide.

Below we have listed some of the problems you can face with mercury 90hp. For your help, we have given adequate solutions to each problem as well. So, what are you waiting for! Jump below and learn how to fix the issue.

1. The Engine Won’t Start

The engine won’t start is the most common problem with any motor or engine. Also, the tricky part is, that it can be caused by anything. From a single screw not being tight to the whole engine being damaged. Anything is possible.

But the most common thing that happens is, that water gets inside the engine. Since engines run on fuel, if any water gets in, it can ruin the engine. The mercury optimax 115 has similar problems as well.

Also, there are fuses inside the engine. The fuse is kind of like a safety switch for the engine. If anything goes wrong while delivering power, the fuse cuts off. If the fuse is broken there will be no power delivery to the propeller.

So, how can we fix this?

The Engine Won’t Start


When the engine is not starting, it’s better to check everything. Anything can cause an engine to break down. Start with checking whether the engine oil is on level or not. Check if there’s water in the carburetor.

If water is mixed with the fuel in the tank, it can cause this engine stoppage as well. Check if the battery is connected to the engine or not. Also, do take a look at the fuses. If you find broken fuses, reconnect with a wire or use a new fuse.

2. Doesn’t hold Speed for Too Long

If the engine is not holding its speed for too long. Most probably the sparkplugs are broken. Or water might have gone near the spark plug. This issue can be also caused by overheating of the engine.

A boat engine does pump in water. One of the reasons is to cool the engine down. Also, to pump water out of the boat.

So, what to do now?


If there are problems with the spark plug, you have to change it. These are really inexpensive parts of the engine. But has a really important role. If water has gone into the plugs, it is broken.

It can also cause overheating in the engine.

3. Fuel Pump Faults

In order to move forward, the engine needs fuel to create energy. There are dedicated fuel pumps that deliver the fuel to the engine. The fuel pump is attached to the pipe which connects the engine to the fuel tank. the pump sits near the engine.

There are rubber gaskets that tighten the pump to the engine. This keeps the fuel not from dripping down out of the pipe. But sometimes the pump fails. These are some of the common issues with the fuel pump. What to do when this happens.


Getting a new fuel pump would be the best thing to do. There are after-market fuel pumps that you can attach to the engine.

Other than the fuel pump, check on the gasket and the clamps. Sometimes the rubber gasket breaks and water mixes up with the fuel inside the engine. Changing the rubber gasket in inspection can solve this problem beforehand.

Fuel Pump Faults

4. Water Mixes with Fuel

From the beginning, we are talking about water mixing with fuel in many different scenarios. That is because it is a common scenario with many boat engines. Especially the mercury 90hp. Although, this is not a problem in mercury optimax 225.

So, how to keep water from mixing with fuel?


Water is kept out of the engine using many different types of gaskets and glues. Also; screws, bolts, and nuts are placed so that water doesn’t get inside. Check if all the screws and gaskets are tight.

This way you’ll know if water is entering the engine or not.

Water Mixes with Fuel

5. The Engine Sits Idle for too long

If the engine is sitting idle for too long, the engine might break down at a point. When you are planning to keep the engine out of use, remember to drain the fuel. If the fuel sits in the engine, it can fumigate and cause damage.

So, how to get fuel out of the engine?


You need to run the engine in order to run the fuel out of the engine. When the engine is running take the hose of the engine which delivers fuel. The engine will auto-stop when the fuel ends.

Afterward, take a flat head and drip the carburetor. There is residual fuel in the carburetor.

And, these are all the common issues people face with the engine.

The Engine Sits Idle for too long

6. Slack in Throttle Gear Mechanism

One common cause of slacking in the throttle gear mechanism is a worn or broken throttle cable. A worn or broken cable can cause the gears to slip and create excessive drag on the engine. In some cases, a broken cable can also cause the engine to stall. Mercury has released a service bulletin (SB) addressing this issue.


This is a common problem on these motors and can be rectified by fixing or replacing the component. If you need to replace the gear, make sure to get a replacement that fits your car exactly, as there may be slight variations in make and model.

If you experience slacking in your throttle gear mechanism, you should take your vehicle to a Mercury dealership for inspection and repair work.

Slack in Throttle Gear Mechanism


1. How to Separate Fuel from Water?

If water has mixed with your fuel, then it’s better to discard that fuel, Also, refuel it again. But if this happens middle of your travel, you have to take the water out. To do that, use a pipe to sip the water out from the bottom of the tank.

2. What to Do when the Engine Stops in the Middle of Traveling?

When your engine stops in the middle of your travel, you have to check the engine. Check all the fuses, fuel intake, fuel level, engine oil, and engine temp. If the engine is running too hot, it can shut down.

3. How to Start the Mercury 90hp 4 Stroke Engine?

In order to start the engine, tilt the engine a bit so that fuel can get in easily. Secondly, prime the bulb of the engine until it’s hard to squeeze. Afterward, while in neutral, move the throttle to ⅔. Lastly, push the choke and crank and releases it to get back to neutral.

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

Mercury engines are designed to run for an average of 3,000 – 4,000 hours. This means they can often last longer than other types of engines. However, this length depends on a variety of factors, including the make and model of the engine, the condition of the vehicle it is installed in, and how it is used.

You can also read: Suzuki 4 Stroke Outboard vs Yamaha


Hopefully, by reading this article you will have a better understanding of what goes wrong with Mercury 90hp 4-stroke engines and be able to troubleshoot the issue yourself if it occurs. By doing so, you can avoid having to take your boat or engine in for repair and save yourself time and money.

If you do experience a problem with your engine, make sure to read through these tips again so that you are fully informed about the potential causes.

That is everything you need to know about, mercury 90hp 4 stroke problems. Always follow the driver’s manual to learn about the engine in depth.

Good luck and have fun out in the sea!

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