Mercury Marine is a well-known manufacturer of marine engines and parts, including the Mercury 90HP 4-stroke outboard motor. While this motor is generally reliable and provides excellent performance, there are a few common issues that owners may experience. In this article, we will cover seven common Mercury 90HP 4 stroke problems, along with their causes and solutions.
Problem #1: Overheating
One of the most common problems with the Mercury 90HP 4-stroke outboard motor is overheating. This issue can be caused by a variety of factors, such as a blocked water intake, a damaged impeller, or a clogged cooling system. If your motor is overheating, you may notice a high-pitched alarm or a reduction in power.
Solution:
To fix an overheating problem, you should first check the water intake to ensure that it is not blocked. You should also check the impeller to make sure that it is in good condition and not damaged. If the impeller is damaged, it should be replaced. You may also need to flush out the cooling system to remove any debris that may be clogging it.
Problem #2: Starting Issues
Another common issue with the Mercury 90HP 4-stroke outboard motor is starting problems. This can be caused by a variety of factors, such as a dead battery, a faulty starter motor, or a clogged fuel system. If your motor is experiencing starting issues, it may crank slowly or not start at all.
Solution:
To fix starting issues, you should first check the battery to ensure that it is fully charged and in good condition. You should also check the starter motor to make sure that it is working correctly. If the starter motor is faulty, it should be replaced. You may also need to clean or replace the fuel filter to ensure that the fuel system is functioning correctly.

Problem #3: Poor Performance
If your Mercury 90HP 4-stroke outboard motor is not performing as well as it should be, there may be several reasons for this. Poor performance can be caused by issues such as a dirty propeller, a blocked fuel line, or a clogged air filter. You may notice a reduction in power or speed, or the motor may feel sluggish.
Solution:
To fix poor performance issues, you should first check the propeller to ensure that it is clean and free from damage. You should also check the fuel line and air filter to ensure that they are not clogged or blocked. If either of these components is dirty, they should be cleaned or replaced.
Problem #4: Fuel Leaks
Fuel leaks are a serious issue that can cause damage to your motor and pose a fire risk. Fuel leaks can be caused by a variety of factors, such as a damaged fuel line, a loose fuel connector, or a faulty fuel pump. You may notice a strong smell of fuel or see fuel leaking from the motor.
Solution:
If you suspect that you have a fuel leak, you should immediately stop using your motor and inspect it for damage. Check the fuel line and connectors to ensure that they are in good condition and not damaged. If you cannot find the source of the leak, you should take your motor to a qualified mechanic for inspection and repair.

Problem #5: Electrical Issues
Electrical issues can be caused by a variety of factors, such as a faulty battery, damaged wiring, or a malfunctioning electrical component. You may notice issues such as a weak or dead battery, erratic electrical behavior, or a complete loss of power.
Solution:
To fix electrical issues, you should first check the battery to ensure that it is fully charged and in good condition. You should also inspect the wiring to ensure that it is not damaged or frayed. If you find any damaged wiring, it should be replaced. You may also need to check the electrical components to ensure that they are functioning correctly. If you are unsure how to perform these checks, you should take your motor to a qualified mechanic for inspection and repair.
Problem #6: Corrosion
Corrosion can occur on any part of your Mercury 90HP 4-stroke outboard motor that is exposed to water or salt. Corrosion can cause damage to your motor, affecting its performance and lifespan. You may notice rust or discoloration on the motor, or you may experience issues such as overheating or poor performance.

Solution:
To prevent corrosion, you should regularly clean and maintain your motor. You should also rinse it with fresh water after every use, particularly if you have been boating in saltwater. If you notice any signs of corrosion, you should address the issue immediately to prevent further damage. You may need to replace the affected components, or you may be able to repair the damage with a rust converter.
Problem #7: Oil Leaks
Oil leaks are another serious issue that can cause damage to your motor and pose a fire risk. Oil leaks can be caused by a variety of factors, such as a damaged oil pan, a loose oil filter, or a faulty oil seal. You may notice oil pooling under your motor or a strong smell of oil.
Solution:
If you suspect that you have an oil leak, you should immediately stop using your motor and inspect it for damage. Check the oil pan, oil filter, and oil seal to ensure that they are in good condition and not damaged. If you cannot find the source of the leak, you should take your motor to a qualified mechanic for inspection and repair.
Wrapping Up
The Mercury 90HP 4-stroke outboard motor is generally reliable and provides excellent performance. However, like any motor, it can experience issues from time to time. The seven common problems outlined in this article include overheating, starting issues, poor performance, fuel leaks, electrical issues, corrosion, and oil leaks.
By understanding the causes and solutions to these problems, you can keep your Mercury 90HP 4-stroke outboard motor in top condition and enjoy many years of trouble-free boating. Remember, if you are unsure how to perform any checks or repairs, you should always seek the advice of a qualified mechanic.