Mercury 40HP 2-Stroke Outboard Water Pump Repair

Servicing the water pump and replacing the impeller in an outboard motor is standard maintenance that should be done every year. 

Replacing the impeller in your Mercury outboard helps ensure your outboard’s cooling system works perfectly, protecting the motor from overheating. Watch the video above and follow the steps below to replace the water pump in a Mercury 40HP 2-Stroke motor. 

Tools and Parts Needed – Mercury 40HP 2-Stroke Water Pump Repair


  • Ratchet and sockets

NOTE: The water pump repair kit includes everything needed to replace the impeller and service the water pump. You can purchase the individual parts separately, but the kit is cheaper. If you prefer to buy the individual parts, they can be found here.

Mercury 40HP 2-Stroke Water Pump Repair

First you’ll need to remove the lower unit from the outboard. Watch the video below to see how to get that done.

Dismantling the Water Pump

Step 1. Remove the four bolts from the water pump housing, then lift the housing up and off the driveshaft.

Mercury 40 water pump parts diagram

Step 2. Remove the impeller and the two nylon washers (one above and one below the impeller).

Mercury 40 water pump impeller removal

NOTE: If the impeller blades are damaged, check whether any pieces have broken off. Find and remove any pieces so they don’t end up blocking the cooling system once the lower unit is reinstalled onto the outboard.

Step 3. Remove the woodruff key under the impeller from the driveshaft.

Step 4. Remove the upper gasket, the metal wear plate and the lower gasket. Carefully scrape away any gasket material from the lower unit if necessary.

Installing the Impeller & Reassembling the Water Pump

Step 5. Install the new lower gasket, the new wear plate and the new upper gasket (all included in the water pump repair kit).

Mercury 40HP 2-stroke water pump parts

Step 6. Install the new lower nylon washer and the new woodruff key (both included in the water pump repair kit).

PRO TIP: Put a dab of marine grease on the driveshaft to keep the woodruff key in place while you install the impeller.

Step 7. Install the new impeller and the upper nylon washer (both included in the water pump repair kit).

Step 8. Lightly grease the metal wear plate and the inside of the water pump housing with marine grease.

Mercury 40 outboard water pump repair

Step 9. Reinstall the water pump housing while rotating the driveshaft clockwise as you press it down over the impeller to ensure the impeller blades are oriented in the correct direction inside the housing. Next, reinstall the four water pump bolts.

Mercury 40HP two-stroke outboard water pump change

Step 10. Install the new water tube seal (included in the water pump repair kit) onto the water pump housing and you’re done.

Replacing the water pump on a Mercury 40HP 2-stroke outboard engine is similar to changing the water pump on a 4-stroke. However, if you want to see it done on a 4-stroke as well, watch the video above to see how to replace the water pump on a Mercury 40HP 4-stroke outboard