Mercury 40HP Outboard Water Pump Replacement

Replacing just the water pump impeller or the entire water pump itself on a Mercury outboard is essentially the same process, as changing out just the impeller requires disassembling the water pump assembly.

It's better to just replace the entire water pump as a whole if you're going to go through the trouble. Watch the video above and follow the steps below to replace the water pump on a Mercury 40 outboard engine.

NOTE: We replaced the water pump on a Mercury 40 outboard, but the steps are the same or similar for most outboard motors.

Tools and Parts – Mercury 40HP Water Pump Repair

• Ratchet
• 10mm socket
• Water pump repair kit
• PTFE marine grease
• Red threadlocker

To access the water pump, you’ll need to remove the gearcase/lower unit from the outboard. Watch our Mercury 40HP lower unit removal video below for reference.

How to Replace Mercury 40HP Water Pump

Step 1. Remove the rubber water tube seal from the top of the water pump assembly.

Step 2. Remove the four bolts that hold the water pump assembly in place, followed by the water pump housing.

Step 3. Remove the impeller and its two nylon washers (the one above it and the one below it).

Step 4. Remove the woodruff key from the base of the driveshaft, followed by the upper gasket, the wear plate and the lower gasket from the gearcase.

NOTE: Inspect the shift shaft boot for cracks or splits and replace it if it’s worn.

Watch the clip below to see the shift shaft boot removal and replacement segment of our Mercury 40HP water pump repair video.

Step 5. Install the new lower gasket, wear plate and upper gasket (all included in the water pump repair kit) with the neoprene side of the upper gasket facing upward.

Step 6. Install the new woodruff key (included in the water pump repair kit) onto the driveshaft, and use a dab of PTFE marine grease to hold it in place.

Step 7. Install the new impeller (included in the water pump repair kit) with the nylon washers above and below it. (The slot in the impeller should sit over the woodruff key).

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

Step 9. Reinstall the water pump housing while rotating the driveshaft clockwise to ensure the impeller blades are turned in the correct direction within the housing.

Step 10. Put a drop of red threadlocker onto each of the four water pump assembly bolts and reinstall them.

Step 11. Install the new the rubber water tube seal (included in the water pump repair kit) onto the top of the water pump assembly and you’re done.

Watch the clip below to see the water pump replacement segment of our Mercury 40HP water pump repair video.

Ready to get the lower unit mounted back onto the engine? Watch the video below to see how to remount the lower unit on a Mercury 40HP outboard.