How to Change a Mercury Outboard Water Pump Impeller

A reliable and efficient functioning water pump impeller is essential for the cooling system of your Mercury outboard engine. Over time, wear and tear can reduce the impeller’s effectiveness, leading to overheating problems that could severely damage your engine. Therefore, knowing how to change a Mercury outboard water pump impeller is essential for boat owners and enthusiasts.

This article offers a step-by-step guide to help you confidently replace the water pump impeller. We break down the process into simple, easy-to-follow stages, making this task less daunting, even for those with limited mechanical experience.

In the video embedded within this article, the water pump impeller change was performed on a Mercury Verado 300. However, the video and the step-by-step instructions included within this article apply to:

• Verado L4 Models, from 135 to 200 HP
• Verado L6 Models, from 200 to 400 HP
• 4-Stroke EFI Models, from 40 to 150 HP

With practical advice, expert tips, and clear instructions, you can soon change your outboard water pump impeller like a pro, ensuring your engine remains cool, efficient, and in top-notch condition.

We also recommend rags or shop towels. Additionally, PartVu’s Mercury water pump repair kits include replacement water pump impellers.

Why Is Inspecting and Replacing an Impeller in a Mercury Outboard Important?

Failure to replace a worn-out impeller can have several serious implications. The impeller, as part of the cooling system in an engine, plays a vital role in ensuring the engine operates at optimum temperatures. When the impeller is worn out, it can’t cool the engine sufficiently, potentially leading to overheating. Overheating can cause significant engine damage, which could incur substantial repair costs.

Moreover, a malfunctioning impeller may also negatively affect the engine’s overall performance. Over time, these issues can compound, leading to decreased lifespan of the engine and other connected systems. In addition to the financial consequences, there’s also a safety risk; a sudden engine failure due to an overheated engine could leave you stranded or in a hazardous situation, especially when navigating on water or in remote areas.

What Is the Recommended Frequency for Servicing the Water Pump Impeller?

The standard Mercury outboard water pump impeller replacement recommendation is every three years or after every 300 hours of operation, whichever comes first. Replacement is an essential preventive measure to ensure the motor operates efficiently and to prevent unexpected system malfunctions.

However, specific symptoms might necessitate an earlier inspection of the impeller. Overheating, decreased water flow from the telltale hole, diminished cooling system efficiency, and reduced engine performance are all signs that the impeller might be worn or damaged. These warning signals are essential to heed as they could help you avoid more serious engine problems in the future.

Sticking to a regular maintenance schedule is crucial to keep the water pump impeller and the entire engine cooling system in optimum condition. But remember, even with a three-year or 300-hour guideline, any abnormalities or potential issues should be addressed as soon as they surface.

How to Change a Mercury Outboard Water Pump Impeller: Step-by-Step Process

You’ll need the following:

• Water pump impeller
• Mercury 2-4-C Grease
• Socket wrenches
• Needle-nose pliers
• Screwdrivers
• Bungee cords
• Rags or shop towels

Regular maintenance on your Mercury outboard water pump is crucial for proper functioning. Here’s a step-by-step guide on how to change the water pump impeller:

1. Remove the Lower Unit (Gear Case)

• Begin by removing the anode.
• Use bungee cords or a suitable method to secure the gear case.
• Remove the mounting bolts.
• Use a rubber mallet to gently tap on the gear case to separate it from the rest of the outboard to avoid damage to the anti-ventilation plate.
• Disconnect the speedo tube using needle-nose pliers. Use a pinch-and-roll method to remove the tube slowly.
• Place the lower unit in a suitable workspace to access the rest of the steps in the process.

2. Remove the Housing

• Next, remove the rubber collar from the top of the water pump assembly.
• Remove the four bolts that hold the water pump assembly in place
• Gently pry off the housing.
• Take note of the condition of the seals for potential replacement.

3. Replace the Impeller

• Use a water pump repair kit suitable for your Mercury outboard motor.
• Remove the old plates and gaskets at the base of the impeller.
• Position the new plates and gaskets using the old ones as a reference for direction and alignment.
• Install the new impeller. Line up the new impeller according to the old one’s orientation.
• Apply grease to the bolts for easier removal in the future.

4. Reassemble

• Reattach the housing. Rotate the driveshaft clockwise to ensure the impeller blades correctly orient within the housing.
• Align the holes with the bolts.
• Gradually tighten the bolts, following torque specifications. Set torque to light setting or hand tighten them with a wrench. Be careful not to overtighten.
• Remember to reinstall the collar washer to prevent water leakage.
• Prepare the gear case studs by cleaning them first before applying grease.
• Carefully maneuver the gear case back into position, aligning the drive shaft first, then the shift shaft. You may have to remove the side cowling to allow easier access to the shift shaft.
• Reconnect the speedo tube using needle nose pliers.
• Secure the gear case by reinstalling the washers and nuts and then tightening the nuts evenly.

Changing a Mercury outboard water pump impeller is a crucial maintenance task that ensures your outboard motor’s longevity and optimal performance. By following the step-by-step instructions in this guide, you can confidently tackle this task and easily replace the impeller.

Remember to gather the necessary tools, pay attention to safety precautions, and take your time to remove the old impeller and install the new one carefully. Regularly inspecting and replacing the water pump impeller will help prevent overheating issues, maintain proper water flow, and ultimately prolong the life of your Mercury Outboard motor.

**Important Note**

Always consult your engine’s owner’s manual for instructions and information specific to your engine. If you need clarification on changing a water pump impeller, consult a Mercury Marine-certified mechanic.

PartsVu and Platinum Mobile Marine Partnership

Thanks to Frank Cutri, the owner and operator of Platinum Mobile Marine, a mobile mechanic service serving the Lee County, Florida area, for partnering with PartsVu on the Boat Maintenance Matters series.

Together, we are working hard to build a library of step-by-step instructions on common outboard motor repair and maintenance tasks.