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Mercury 25 Hp 4 Stroke Problems – 4 Problems and Solution

Mercury outboards are known for their reliability and utility. But they may face some problems as well which is common. Similarly, there are many problems with Mercury 25 hp 4 stroke engines.

But what are the Mercury 25 hp 4 stroke problems?

There might be various problems in Mercury 25 hp 4 strokes. For example, idle outboard, acceleration problems, power loss issues, etc and a lot. There are these problems and also some possible fixes for them. However, knowing about them specifically helps a lot to get faster relief.

This short overview here might not fulfill your cause. But don’t be afraid, we have a full-size discussion to let you know everything in detail. Keep reading along if you have time to spare.

4 Problems of Mercury 25 Hp 4 Stroke Engines

We’ve selected 4 Mercury 25 hp 4 stroke problems for our discussion. These problems are pretty common and most of the users deal with them. Along with the problem, we’ve tried to give some probable solutions that might help. Also, check our article and find our comparison of the Volvo Penta vs Mercruiser.

So, let’s get started with them one by one.

Problem 1: Rough Idleness of the Outboard

Many users complain about this problem. According to them, the Mercury outboard motor idles roughly. Not only that; but sometimes it stalls, and users face issues with the Mercury voltage regulator. As a result, irregular performance is noticed from the outboard motor.

This problem may occur due to various reasons. Some of them are as follows:

  • Low Pressure of Fuel Pump (Lower pressure of the fuel pump won’t let the proper amount of fuel be injected into the motor. Thus, the motor won’t perform well.)
  • A closed or restricted air vent in the fuel tank.
  • Fuel filter damage or fuel line restriction


Tests can be done to inspect the problem and fix it accordingly. Follow the instructions provided in the repair guide by the manufacturer.

First, you should disconnect the motor from the power source. Gradually you should remove the cowl cover followed by plugs, and spark plug wire boot.

After that, check if the compression gauge is working properly. It should give a reading of 30 PSI. Anything below or above this value can lead to the replacement of the gauge.

Also, check if the diaphragm is damaged or not as well as one-way valves. If there are any faulty parts, you should replace them.

Problem 2: Hesitation of Engine Due to Acceleration

Hesitation of Engine Due to Acceleration

Sometimes the engine runs unevenly and also struggles to accelerate. There are several reasons for this issue. Two of them are-

  • Lack of spark or weak spark in the ignition system
  • No spark in the carburetor


Use a spark tester for testing the ignition system. There is no specific spark tester for the Mercury 25-hp 4-stroke engine. So, any of the spark testers would do.

If you don’t see any spark while testing, you might need to change the spark wires. This may be needed in the case of poor or weak spark as well. For changing the spark wires, you’ll need to remove them from the switch box.

For the carburetor issues, there’s a different solution. First, you should check the carburetor spark with the same spark tester. If you find it faulty, then you might need to clean the carburetor.

For that, remove all the old, dirty fuel from the carburetor. Then dip the carburetor in a cleaning solution without removing the main jet. After soaking for around an hour, you can consider removing the main jet from the carburetor.

After removing the jet, clean it separately. In case you find it damaged, replace the jet with a new one.

Problem 3: Power Loss Issue

Power Loss Issue Mercury

You can experience power loss at times. But when it becomes unusual, that’s when the problem arrives. The engine sputters and constantly loses power due to this problem.

This may happen if you leave your outboard unused for a long time. But there can be other reasons such as bad fuel conditions and more.


Every outboard motor has a recommended fuel to use for the best performance. So, if that’s not maintained, a problem is bound to occur.

So, use the recommended octane fuel for your Mercury 25 hp 4 strokes outboard engine. Also, change or clean the fuel on a regular basis. You can consider using a high-quality fuel and water-separating filter. A fuel stabilizer may also help in this regard. Mercury 20 Hp 4 Stroke also has that problem.

Problem 4: Faulty Engine Oil Pressure Sensor

This is a common issue with these engines and can result in poor performance and even engine failure.

If you’re experiencing any of the following symptoms, it’s time to take your mercury out for servicing: low engine oil pressure, rough idle, poor acceleration, or loss of power. The faulty sensor is most likely the cause of these issues and can be replaced without any difficulty


This issue can be remedied by replacing the sensor. Here’s how:

1. Remove the spark plug cover.
2. Disconnect the oil pump pickup tube from the engine block (3).
3. Remove the oil pan gasket (4) and bolts – take care not to lose any of the oil that is in the pan.
4. Lift off the pan and check for evidence of a broken or cracked pressure sensor (5). If so, replace it with a new one and re-install in reverse order.
5. Replace the pan gasket and bolts and reinstall the sparkplug cover.

So, these were the 4 problems we discussed the Mercury 25 hp 4 stroke motor.

User Acknowledgements

Mercury 25 hp 4 stroke outboard is really a popular and widely used outboard engine. This motor works very well in different conditions. That’s why most of the users are satisfied with the quality performance they get.

Mercury 25 hp 4 stroke outboard

But it’s also true that they’re bound to face some issues in the long run. Fortunately, most of them are easy to fix. So, the users don’t go through a lot of hassle.

However, maintaining safety is a must while doing any work. So, take all the safety measures while working on them. Maybe you are interested in problems with Mercury Optimax 115.


1. Can you rely on Mercury 25 hp 4 stroke?

Yes, they are very reliable due to their 4-stroke functioning. Also, they are very fuel-efficient and generate less noise. However, they are comparatively heavier than other engines.

2. Is a Mercury 25 hp 4 stroke outboard fast enough?

Mercury 25 hp 4 stroke outboards can go up to 15-20 mph speed. It depends on the loading conditions. However, it can reach almost 25 mph in no-loading conditions. So, it can be said that they have decent speed.

Mercury 25 hp 4 strokes can be operated for 3,000 to 4,000 hours. However, this can vary depending on how well the outboard is maintained. Poorly maintained outboards won’t last this much.

4. When is the right time to change the oil in a Mercury 4 stroke outboard?

You should change the oil after every 100 hours. Or every year for most of the 4 stroke outboards. This is recommended by the experts to do so.

3. How many hours can Mercury 25 hp 4 strokes last?


Despite being a trusted brand, the minor issues occurring in a Mercury outboard are discussed here. So, this was all about Mercury 25 hp 4 stroke problems and their solutions.

Make sure to take care of your outboard properly and maintain it well. That’s all from us. Have a nice day.

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