Is your Mercury outboard water pump not cooling the engine like it used to? A bad outboard water pump can trigger serious performance problems, including overheating, restricted RPMs, or worse: significant engine damage if left unchecked. Most of the time, these water pump troubles boil down to a faulty impeller. In this article, we’ll explore:

• How to Tell If Your Outboard Water Pump Is Bad
• How Often You Should Change Your Mercury Outboard Water Pump Impeller
• Step-by-Step Guide to Replacing a Mercury Outboard Water Impeller
• Quick Bilge Pump Maintenance Tips
• Estimated Costs for Water Pump Impeller Replacement
• Final Thoughts

How Do I Know If My Outboard Water Pump Is Bad?

A malfunctioning Mercury outboard water pump may show warning signs like:

1. Overheating
   An overheating engine (particularly in the 120–140°F range at idle) is often the first clue. When the temperature alarm sounds or temperatures climb above normal, it’s time to shut down the motor and investigate. Other causes of overheating may include a faulty thermostat, blocked exhaust manifold, or a worn impeller.
2. Reduced Outboard Performance
   If your boat “bogs down” and won’t reach normal RPMs, a failing water pump may be to blame. However, many other factors (propeller issues, bad spark plugs, clogged fuel filters) can also cause similar problems. Rule these out before focusing on the water pump.
3. Reduced Water Stream from the Tell-Tale Hole
   The tell-tale hole should emit a clear, strong stream of water while the motor is running. A weaker flow can indicate blockages or a damaged water pump housing/impeller. Insert a thin wire to ensure the tell-tale outlet isn’t clogged by debris, salt buildup, or insects.
4. Lack of Regular Maintenance (Worn Impeller)
   Mercury outboards generally require maintenance every 100 hours (annually) and more comprehensive servicing every 300 hours (about every three years). Skipping these intervals can accelerate impeller wear and reduce cooling performance.

How Often Should You Change the Water Pump on a Mercury Outboard?

• In most cases, only the impeller needs replacement rather than the entire water pump assembly.
• Every 3 years (or 300 hours) — whichever comes first — is a good rule of thumb for changing the impeller. If you’re encountering recurring cooling issues after replacing the impeller, you may need a complete water pump replacement.

How to Replace Your Mercury Outboard Water Impeller

Although Mercury outboard models vary, the following steps provide a general idea for most engines (e.g., a 60HP Mercury):

Tools Needed

• Ratchet and socket (10mm for many models)
• Marine grease

Parts Needed

• Mercury outboard water pump kit (including impeller, gaskets, and seals)

Steps

1. Remove the Lower Unit
   Unbolt and carefully lower the gearcase from the outboard.
2. Take Off the Old Pump Components
   Remove the rubber water tube seal. Unbolt and lift off the water pump housing. Slide off the impeller and inspect the gaskets.
3. Replace Gaskets & Impeller
   Remove the wear plate and any old gaskets. Insert the new lower gasket, wear plate, and upper gasket. Install the new impeller, ensuring it sits correctly on the woodruff key.
4. Reinstall the Pump Housing
   Apply marine grease inside the housing. Slide the housing down while rotating the driveshaft clockwise to properly seat the impeller fins. Bolt everything back together securely.
5. Reattach the Lower Unit
   Carefully align and reconnect the gearcase. Tighten all bolts according to the recommended torque specs in your owner’s manual.

By following these steps, you’ll restore consistent water flow to keep your outboard cool and protect your engine from potential harm.

Quick Tip: Bilge Pump Maintenance

While focusing on your water pump is essential, don’t overlook your bilge pump — it’s another critical part of your boat’s overall health. Here’s a quick annual maintenance checklist, adapted from Mercury’s Quick Tip video, to keep your bilge pump working effectively:

1. Locate and Access the Bilge Pump
   Typically near the centerline and the lowest point of the hull, close to the drain plug.
2. Turn Off the Bilge Pump
   Disconnect it from the boat’s wiring harness.
3. Remove Debris
   Clear any leaves, grass, or trash by hand.
4. Remove the Discharge Hose
   Use a screwdriver to loosen the hose clamp.
5. Inspect and Clean the Pump
   Twist the top of the pump counterclockwise to remove it from its base. Cut away any fishing line or debris from the impeller. Check for cracks or breakage in the impeller—replace if damaged.
6. Vacuum and Scrub
   Use a wet/dry vac to clear the bilge of standing water and debris. Rinse the pump with vinegar, scrub away mold or sediment, and then rinse with fresh water.
7. Grease the O-Ring
   Apply a thin coat of grease before reassembling.
8. Reinstall Everything
   Reattach the discharge hose, tighten the clamp, and reconnect the wiring harness.
9. Test the Pump
   Fill the bilge with a few gallons of water and ensure the automatic float switch (if present) activates. Test the manual switch as well, and don’t let the pump run dry once the water is cleared.

A little attention each year can greatly extend the life of your bilge pump while also providing you with a crucial early-warning system if your boat develops a leak.

How Much Does It Cost to Replace a Water Pump Impeller on a Mercury Outboard?

• Expect to pay $15–$35 for a replacement impeller, depending on the Mercury outboard model.
• A complete water pump kit (which includes gaskets, seals, and wear plate) typically costs more but is worth the investment to avoid missing crucial components.

Final Thoughts

Regular upkeep on both your Mercury outboard water pump and bilge pump is crucial for safe and enjoyable boating. By inspecting and replacing the impeller every three years and giving the bilge pump a quick cleaning at least once a year, you’ll prevent minor issues from snowballing into major repairs.

Happy Boating and Safe Travels!