How to Replace a Boat Impeller: DIY Guide with Pictures

Why Impeller Replacement Matters

Your impeller is a small rubber wheel with flexible blades that pumps cooling water through your engine. When it fails, your engine overheats - sometimes in minutes. The good news? Replacing it is one of the easiest DIY jobs on an outboard.

When to Replace Your Impeller

• Every 2-3 years as preventive maintenance
• Every year if you run in sandy or shallow water
• Immediately if you notice weak tell-tale flow
• After any instance of running the engine dry
• If you find rubber pieces in the tell-tale stream

Tools You'll Need

• Socket set or wrenches (sizes vary by brand)
• Screwdrivers (Phillips and flat)
• Needle-nose pliers
• Impeller puller (optional but helpful)
• New impeller kit (includes gaskets and O-rings)
• Waterproof grease
• Clean rags

Step-by-Step: Replacing Your Impeller

Step 1: Gather Information

Find your engine's year, make, model, and serial number. You'll need this to get the correct impeller kit. Most kits are $20-50 and include the impeller, gasket, and necessary O-rings.

Step 2: Access the Lower Unit

On most outboards, you'll need to remove the lower unit to access the water pump. This typically involves:

1. Removing the shift linkage
2. Removing 4-6 bolts holding the lower unit to the mid-section
3. Carefully lowering the lower unit (support it - they're heavier than they look)

Step 3: Remove the Water Pump Housing

With the lower unit removed:

1. Remove the bolts holding the water pump housing
2. Carefully lift off the housing
3. Note the orientation of the old impeller
4. Remove the old impeller (may need a puller if it's stuck)

Step 4: Inspect the Housing

Look at the inside of the water pump housing (the stainless steel cup). Check for:

• Deep grooves or scoring
• Rough spots or corrosion
• Proper shape (should be perfectly round)

If the housing is damaged, replace it. Running a new impeller in a scored housing drastically shortens its life.

Step 5: Install the New Impeller

1. Coat the new impeller with waterproof grease
2. Note the direction of blade curl (usually marked or visible from the old one)
3. Slide the impeller onto the drive shaft
4. Rotate the shaft while pushing down to help the blades fold correctly
5. Install the new gasket (usually goes under the housing)
6. Replace any O-rings included in the kit

Step 6: Reassemble

1. Reinstall the water pump housing
2. Torque bolts to spec (check your manual)
3. Reinstall the lower unit
4. Reconnect the shift linkage
5. Refill the lower unit with gear oil

Step 7: Test

Run the engine on a hose (muffs) and verify:

• Strong, steady tell-tale stream
• No leaks around the water pump area
• Proper shift operation

Common Mistakes to Avoid

• Wrong impeller direction - Blades must curl in the correct direction for your engine's rotation
• Dry installation - Always use grease; dry rubber tears on first start
• Forgetting the key - The small woodruff key that locks the impeller to the shaft is easy to lose
• Over-torquing - Use a torque wrench; stripped threads are expensive

Time and Cost

• DIY cost: $20-50 for the kit
• DIY time: 1-2 hours
• Shop cost: $150-300+

This is one repair that absolutely pays to do yourself. Save the money for something you can't fix!